A study of temporal and spatial variations in the polar cusp/cleft region using multi-instrument techniques

A study of temporal and spatial variations in the polar cusp/cleft region using multi-instrument techniques

by

Kjellmar Oksavik

A thesis submitted to the Department of Physics, University of Bergen, in partial fulfilment of the requirements for the degree of Doctor Scientiarum

Bergen October 2002

University of Bergen University Courses

on Svalbard

Acknowledgements

This thesis is the result of a three-year Dr. Scient. grant from the Space Research CLUSTER program of the Norwegian Research Council. Additional travel grants have been provided by the Norwegian Physical Society, American Geophysical Union, John Hopkins University, Scientific Committee on Solar-Terrestrial Physics, Local organizing commettee of S-RAMP 2000 meeting in Sapporo in Japan, and Air Force Office of Scientific Research (AFOSR) under the Window on Science Program.

The Dr. Scient. project has been a cooperation between the University of Bergen (UiB) and the University Courses on Svalbard (UNIS). The first six months (January 1 to June 30, 2000) I stayed at UNIS, and the remaining 2.5 years I stayed at UiB. The University of Oslo (UiO), Boston University (BU), and the European Incoherent SCATer (EISCAT) Scientific Association have also been essential contributors to the project.

I am therefore happy to acknowledge support from a long list of individuals. First of all I would like to thank my two excellent advisors Finn Søraas (UiB) and Jøran Idar Moen (UiO/UNIS), who proposed (in 1998) and reproposed (in 1999) the project to the Norwegian Research Council. Furthermore, Herbert C. Carlson (AFOSR/UiO) has been a true source of inspiration and good ideas. I am also appreciating the fruitful discussions with Theodore A. Fritz, Quigang Zong, and Berend Wilken

during my three months (August 1 to October 31, 2001) at BU. David S. Evans (NOAA Space Environment Center), Robert Pfaff (NASA/Goddard Space Flight Center), and William J. Burke and William F. Denig (Air Force Research Laboratory, Hanscom) have been of great help over several years. Mark Lester (University of Leicester) provided me with data from the Co-ordinated UK Twin Located Auroral Sounder System (CUTLASS). At the EISCAT Svalbard radar the knowlegde of Anthony P.

van Eyken and Rob Dickens have been essential in order to implement our scan modes, and the detailed knowlegde on EISCAT data analysis of Patrick Guio (UiO) has been greatly appreciated. I am also pleased to acknowledge Bjørn Lybekk (UiO) for allowing me to run EISCAT data analysis around the clock the last 2-3 years.

Finally, I would also like to thank the EISCAT Svalbard Radar staff, and all the staff and students at UiB/UNIS/UiO/BU.

Bergen, October 2002 Kjellmar Oksavik

† Deceased.

Contents

Introduction ... 1

Summary of Papers ... 8

Future Prospects ... 10

References ... 11

Paper 1

Optical and particle signatures of magnetospheric boundary layers near magnetic noon: Satellite and ground-based observations,

K. Oksavik, F. Søraas, J. Moen, and W. J. Burke, J. Geophys. Res., 105 (A12), 27555, 2000.

Copyright 2000 American Geophysical Union.

Paper 2

Three-dimensional energetic ion sounding of the magnetopause using Cluster/RAPID, K. Oksavik, T. A. Fritz, Q.-G. Zong, F. Søraas, and B. Wilken,

Geophys. Res. Lett., 29 (9), 1347, doi:10.1029/2001GL014265, 2002.

Copyright 2002 American Geophysical Union.

Paper 3

ESR mapping of polar-cap patches in the dark cusp,

H. C. Carlson, K. Oksavik, J. Moen, A. P. van Eyken, and P. Guio, Geophys. Res. Lett., 29 (10), 1386, doi:10.1029/2001GL014087, 2002.

Copyright 2002 American Geophysical Union.

Paper 4

Simultaneous optical, CUTLASS HF radar, and FAST spacecraft observations:

Signatures of boundary layer processes in the cusp,

K. Oksavik, F. Søraas, J. Moen, R. Pfaff, J. A. Davies, and M. Lester, Submitted to Ann. Geophys., 2002.

*Papers 1-3 are reproduced by permission of American Geophysical Union given by Michael Connolly (Journal Publications Specialist) on September 30, 2002.

Introduction

Some of the fundamental processes in magnetospheric physics are; how particles, momentum, and energy are extracted from the solar wind and transferred to the magnetosphere-ionosphere system. These processes are closely linked to spatial and temporal variations in particle precipitation and current systems in the polar cusp/cleft region. In order to understand boundary layer physics it is essential to determine the magnetic field topology of the front side magnetosphere where this energy transfer takes place. A crucial part of this study is to identify the open/closed field line boundary both near the magnetopause and in the ionosphere. An important step is to study source mechanisms for particles and energy more precisely than previously.

The complex matter of studying our Earth’s outer environment has generally been restricted to obtaining only one-dimensional views by collecting data simultaneously from one, or at best two, spacecraft in the same region [Stern, 1996]. However, during the International Solar-Terrestrial Physics Program several spacecraft have been launched to obtain coordinated simultaneous investigations of the Sun-Earth space environment over an extended period of time. Upstream of Earth the Wind spacecraft [Harten and Clark, 1995] is forecasting the solar wind. Near the magnetopause, where the interaction between the solar wind and the terrestrial magnetic field takes place, CLUSTER [Escoubet et al., 1997] gives unique observations using four identical spacecraft in a tetrahedron formation as shown in Figure 1. This European mission represents the most advanced in-situ scientific instrumentation ever flown in space and provides new opportunities for three-dimensional studies of variations in time and space. At mid-altitude (~4000 km) the high-resolution FAST spacecraft [Carlson et al., 1998] studies the transfer of energy and momentum, and at low-altitude (~800

Figure 1: Artist’s impression of the four Cluster spacecraft (Rumba/Salsa/Samba/Tango) orbiting Earth in a tetrahedron formation [European Space Agency © 2000].

km) the NOAA [Raben et al., 1995; Evans and Greer, 2000] and DMSP [Hardy et al., 1984; Rich and Hairston, 1994] spacecraft give the energy deposit in the ionosphere.

From the ground the ever-changing space weather is reflected in auroral emissions that can be studied for example from the Svalbard stations in Longyearbyen and Ny Ålesund. These two optical sites are perfectly located under the dayside auroral oval providing unique observational conditions during the long polar night. Independent of cloud coverage the ionospheric response and polar cap dynamics can be studied with radars like the EISCAT Svalbard radar [Wannberg et al., 1997] and the SuperDARN/CUTLASS HF radars [Greenwald et al., 1995; Milan et al., 1997]. Most data sets are now distributed worldwide on the Internet in near real-time.

The present study takes advantage of such multi-instrument spacecraft, radar, and ground optics data providing great opportunities for verifying of existing knowledge and exploring of new phenomena in space physics. Such testing is indeed needed, as illustrated in Paper 1 in this thesis, where two different instrumental techniques give apparently inconsistent results (DMSP F13 indicates open magnetic field lines, while NOAA 12 indicates closed field lines in the low-latitude boundary layer) calling for more research. Therefore, in Papers 2, 3, and 4, new techniques are developed using multi-instrument data to examine the open/closed field line boundary and the interaction between the solar wind and the magnetosphere-ionosphere system.

Interaction between the solar wind and the magnetosphere may take place on open (magnetic reconnection) or closed (viscous interaction) magnetic field lines. [Dungey, 1961] proposed that the interplanetary and Earth’s magnetic field could join through magnetic reconnection. According to Crooker [1979, 1992] magnetic reconnection at the magnetopause is most likely to occur at points where terrestrial field lines are anti- parallel to the interplanetary magnetic field (IMF). Southward IMF (B

<0) favors low- latitude (sub-solar) reconnection on the dayside, while northward IMF (B

>0) favors high-latitude (lobe) reconnection. Viscous interaction [Axford and Hines, 1961] for particle and momentum transfer across the magnetopause includes particle diffusion in wave-particle interactions, large-scale fluid interactions (e.g. Kelvin-Helmholtz instability), solar wind pressure variations, and impulsive plasma penetration [e.g., Ma et al., 1991; Lui et al., 1989; Cowley, 1984; Lemaire et al., 1979]. Viscous coupling is only thought to be of relative importance during periods of northward IMF.

From particle characteristics observed by spacecraft several regions of the

magnetosphere have been identified, as shown in Figure 2. In this study we will focus

on the dayside magnetosphere and the regions cusp, plasma mantle, and low-latitude

boundary layer (LLBL), since these regions play a key role for the coupling between

the solar wind and the polar ionosphere. Newell and Meng [1992, 1994] used particle

observations from low-altitude (~800 km) satellites to map the statistical probability

of observing particle precipitation from different magnetospheric regions in the

dayside ionosphere at a given latitude and magnetic local time. Figure 3 shows

statistical locations of the cusp, plasma mantle, LLBL, central plasma sheet (CPS),

and boundary plasma sheet (BPS). The determination criteria are based on

characteristics of the particle precipitation [Newell and Meng, 1988]. The cusp is

characterized by high fluxes of magnetosheath plasma (i.e. ~100 eV electrons and ~1-

3 keV ions). LLBL particles have higher energies and ~10 times lower spectral flux

than in the cusp, consisting of a mixture of magnetosheath and magnetospheric

plasma. Plasma mantle particles have lower energies than cusp particles, with low density, temperature and flow velocity.

For southward IMF several authors have suggested the LLBL, cusp and mantle are entirely on open field lines [Onsager and Lockwood, 1997; Lockwood, 1998], at least near noon [Newell and Meng, 1998]. On anti-sunward convecting field lines in the

Figure 3: A statistical map of the dayside ionosphere showing where particle precipitation from different magnetospheric regions is most likely to be observed [Newell and Meng, 1992].

Figure 2: The magnetosphere [after Siscoe, 1991].

cusp region the low-energy ion cutoff or “staircase” cusp signature is regarded as a signature of magnetopause reconnection and open field lines [e.g., Newell and Meng, 1991, 1995; Lockwood et al., 1998] explained by E × B filtering of precipitating ions [Onsager et al., 1993]. Cusp ion steps are predicted by the Cowley-Lockwood model of flow excitation [Cowley, 1998] and have been simulated by pulsating cusp models [Lockwood and Smith, 1994; Lockwood and Davis, 1995; Smith and Lockwood, 1996]. An alternative approach has been to use the 30 keV electron trapping boundary to identify the open/closed field line boundary [e.g., Moen et al., 1996, 1998, 2001b].

Energetic ions cannot always be used to identify the open/closed field line boundary, since these may exist on open field lines [Lockwood and Moen, 1996]. In Paper 1 a case study is presented indicating a weakness with the two established techniques of low-energy ion cutoff and electron trapping boundary identification.

From ground the high-latitude dayside aurora was first studied from the Russian stations Barentzburgh and Pyramiden on Svalbard [e.g., Feldstein and Starkov, 1967].

A decade later Vorobjev et al. [1975] observed poleward moving auroral forms (PMAFs). The direction of motion of these PMAFs is controlled by the polarity of IMF B

[Sandholt et al., 1986, 1993; Moen et al., 1999]. Poleward moving transients are also seen by coherent-scatter HF radars, and depending on their characteristics, they have been termed; flow channel events (FCEs) [Pinnock et al., 1993, 1995;

Neudegg et al., 1999, 2000], pulsed ionospheric flows (PIFs) [Provan et al., 1998;

Provan and Yeoman, 1999; McWilliams et al., 2000], or poleward moving radar auroral forms (PMRAFs) [Milan et al., 2000; Wild et al., 2001]. These different transients are often related to each other [Wild et al., 2001], and dayside transients typically show repetition rates [e.g., Milan et al., 1999] comparable to that of flux transfer events (FTEs) at the magnetopause [Russell and Elphic, 1978; Haerendel et al., 1978]. Several authors have used HF backscatter to identify the cusp [e.g., Moen et al., 2001a, 2002], and the cusp behaves like a hard target for HF backscatter [Milan et al., 1998]. Generating mechanisms for these field-aligned irregularities [Greenwald et al., 1995] are still open questions [Moen et al., 2002]. In Paper 4 we present a case study consistent with a proposal of André et al. [1999] and suggesting a phenomenological relationship between HF radar spectral width enhancements (the signature of cusp [e.g., Moen et al., 2001a]) and electric field turbulence on open field lines [e.g., Maynard et al., 1991; Basinska et al., 1992; Erlandson and Anderson, 1996; Tsurutani et al., 1998; Pfaff et al., 1998] seen by the FAST spacecraft in 4000 km altitude.

Changes in the IMF are often observed as a shift in the location of the cusp aurora [e.g., Moen et al., 1999], and Sandholt et al. [1998] have classified the dayside aurora into various types that depend on the IMF orientation and the magnetic local time of observation, see Figure 4. Papers 1 and 4 focus on the Type 1 and 2 auroral forms near magnetic noon that appear to the south (north) of Svalbard for periods of southward (northward) IMF [Sandholt et al., 1996a, b; Øieroset et al., 1997]. Field- aligned currents (FACs) are flowing in the dayside ionosphere [Iijima and Potemra, 1976a, b; Erlandson et al., 1988; Bythrow et al., 1988; Stauning et al., 2001; Woch et al., 1993]. Little is, however, known about FACs and the current carriers in the cusp region [Yamauchi et al., 1998], as discussed in Paper 4. Furthermore, the LLBL is often found to contain counterstreaming or bi-directional field-aligned streams of electrons with energies of 20-500 eV [e.g., Sharp et al., 1980; Zanetti et al., 1981;

Collin et al., 1982; Ogilvie et al., 1984; Lundin et al., 1987; Farrugia et al., 1988;

Klumpar et al., 1988; Hultqvist et al., 1991; Hall et al., 1991; Traver et al., 1991;

Fuselier et al., 1995, 1997; Sauvaud et al., 1997; Miyake et al., 1998; Yoshioka et al., 2000]. Counterstreaming electrons in the LLBL have often been interpreted as an evidence for closed field lines [e.g., Traver et al., 1991]. Other authors [e.g., Lockwood et al., 2001] have suggested that counterstreaming electrons can be viewed as evidence for open LLBL, and Paper 4 shows an example supporting this interpretation. Paper 4 also suggests that these counterstreaming electrons may be an important source for 630 nm auroral emissions in the cusp region.

Another effect of the Sun-Earth connection is the appearance of polar cap patches [Crowley, 1996; Dandekar and Bullett, 1999; Basu and Valladares, 1999] that dominate the polar cap ionosphere during periods of southward IMF. Polar cap patches are 100-1000 km wide islands of high-density F-region plasma surrounded by significantly lower density plasma. Plasma density inside and outside the patches may in extreme cases switch between 10

m

to 10

m

[Weber et al., 1984]. Patches move antisunward across the polar cap at velocities of ~1 km/s from near noon toward midnight consistent with the two-cell convection pattern [Heelis et al., 1982; Heppner and Maynard, 1987], exit from the nightside polar cap [Pedersen et al., 2000], and return towards noon in subauroral sunward flow. Patches are highly structured over scale sizes of 0.1 to tens of km, giving severe scintillation effects [Buchau et al., 1985; Basu and Valladares, 1999] on GPS navigation and satellite communication signals [Basu et al., 1987; 1994]. It is, however, not known why patches exist, and the dominant mechanisms for patch formation are still unknown [Carlson, 1994; Basu and Valladares, 1999]. Several mechanisms have been suggested; either due to fluctuations in the IMF, or due to transient changes in production, loss, and/or transport in the ionospheric plasma equation. Two of the most prominent mechanisms at present are sporadic chopping of a preexisting tongue of high-density plasma

Figure 4: A classification of dayside auroral forms for different IMF orientations as a function of magnetic local time and latitude [Sandholt et al., 1998].

[Valladares et al., 1994, 1996, 1998, 1999] and sporadic injection of high-density plasma [Lockwood and Carlson, 1992]. The former mechanism will lead to density bite-outs or depletions in a preexisting tongue of high-density plasma entering the polar cap from noon collocated with jets of high plasma velocity, and the latter will

Figure 5: Electron observations from the PEACE instrument on the Cluster spacecraft Rumba (top), and spacecraft potential on Rumba (middle), and a sequence of six all-sky images from 13:31:30 to 13:36:30 UT with the Cluster footprint overlaid (bottom) [after Moen et al., 2001b]. The sounding interval presented in Figure 1 of Paper 2 is indicated with pink dashed lines.

lead to density enhancements associated with high plasma velocity flows. The Valladares mechanism forms patches by carving channels through chemical loss to segment plasma tongues, and the Lockwood-Carlson mechanism forms patches through transient magnetopause reconnection migrating the merging gap equatorward during reconnection pulses and relaxing the open/closed boundary back to its equilibrium position between reconnection events. There are unfortunately few available experimental data sets available for testing the various mechanisms. Paper 3 presents a newly developed incoherent scatter radar mapping technique allowing the observation of polar cap patches in the dark cusp with the needed high temporal and spatial resolution. Similar techniques have so far only been operated in the sunlit cusp [Nilsson et al., 1996, 1998, Valladares et al., 1994, 1998, 1999].

Three-dimensional energetic particle distributions near the magnetopause [e.g., Williams, 1979; Williams et al., 1979, Eccles and Fritz, 2002] can be used to estimate the orientation and velocity of the magnetopause in a direction perpendicular to the magnetic field using a technique of Fahnenstiel [1981]. The same method has later been in use by others [Fritz and Williams, 1984; Zong et al., 2000]. There is, however, a limitation to this technique; it has only been used for a single spacecraft near the Equator where magnetic field geometry is convenient. In Paper 2 the technique is extended so that it works for multiple spacecraft data at high latitudes (i.e. any magnetic field orientation), and the technique is applied on a data set from the imaging energetic particle spectrometer RAPID [Wilken et al., 1997, 2001] during a high-latitude Cluster crossing of the magnetopause on 14 January 2001. Zong et al.

[2001] present an overview of this RAPID data set, and Opgennorth et al. [2001]

discuss the geophysical context in great detail, and it is believed that this boundary

crossing is Cluster’s first encounter with the cusp. Moen et al. [2001b] combine

Cluster observations with ground-based optics from Svalbard, and in order to provide

the reader with a useful background for Paper 2, we would like to summarize some of

their results. The top panel of Figure 5 shows a color-coded electron spectrogram

from the PEACE instrument [Johnstone et al., 1997] on the Cluster spacecraft Rumba,

and the middle panel is a plot of the spacecraft potential (i.e. proportional to the

electron density surrounding Rumba). Shown with pink dashed lines is the time period

of the energetic ion sounding in Paper 2. From 13:27 to 13:57 UT the Rumba

spacecraft entered a region of soft magnetosheath electrons with energies less than

200-300 eV. Around 13:31 UT and in the interval 13:40-13:45 UT there were two

maximums in the electron flux, and the spacecraft potential indicated high electron

density peaking during the first maximum. The bottom image frames of Figure 5

show a sequence of simultaneous cusp auroral observations, with Cluster west of

Svalbard (just outside the all-sky camera field-of-view). Yellow and red lines mark

the Cluster trajectory using solar wind time lags of 60 and 75 minutes, respectively

(see Moen et al. [2001b]). As discussed by Moen et al. [2001b] the cusp aurora started

to intensify and drift in from west after Cluster encountered the region of soft

magnetosheath electrons. In Figure 1 of Paper 2 it is shown how the energetic ion

sounding can be used to study the motion of the magnetopause. It is seen that the

Rumba spacecraft (black lines) briefly crossed the magnetopause around 13:32 UT

and around 13:40-13:42 UT, and throughout the 13:27-13:57 UT interval the

energetic ion sounding technique indicated that Rumba stayed less than one ion gyro

radius inside the magnetopause.

The purpose of this thesis has been to establish and examine multi-instrument techniques and to obtain a better understanding of the different signatures in the dayside aurora, with special emphasis on the role and the dynamics of the open/closed field line boundary. This Dr.Scient. project has resulted in ten papers. However, only four papers have been selected for presentation, since these papers have in common the use of multi-instrument techniques to study processes near the open/closed field line boundary of the dayside magnetosphere and ionosphere.

Summary of Papers

Paper 1: Optical and particle signatures of magnetospheric boundary layers near magnetic noon: Satellite and ground-based observations, K. Oksavik, F. Søraas, J.

Moen, and W. J. Burke, J. Geophys. Res., 105 (A12), 27555, 2000.

A set of satellite and ground-based observations is presented, and this data set suggests that energetic electrons from the magnetosphere cannot be used as an unambiguous discriminator between open and closed magnetic field lines on the dayside. Data sets from the DMSP F13 and NOAA 12 spacecraft flying through dayside Type-1 cusp aurora (both close in time and space) are used to reach two apparently incompatible conclusions. Cusp/mantle precipitation, stepped cusp signatures, and antisunward convection in the DMSP F13 data set strongly suggest open magnetic field lines. On the other hand, NOAA 12 observed a mixture of both magnetosheath and isotropic energetic particles.

Trapped energetic electrons are traditionally regarded as being on closed flux.

In addition to earlier proposed trapping on open field lines, this paper suggests that transmission lines connecting merging sites near the cusp in the Southern Hemisphere with the Northern Hemisphere auroral ionosphere can be several tens of R

long. Alfvén wave transit times of several minutes may introduce ambiguity problems whether or not satellite measurements in the ionosphere are on open or closed magnetic field lines. The paper calls for new research tools to unify the understanding of complementary particle measurements from the DMSP and NOAA satellites.

Paper 2: Three-dimensional energetic ion sounding of the magnetopause using Cluster/RAPID, K. Oksavik, T. A. Fritz, Q.-G. Zong, F. Søraas, and B. Wilken, Geophys. Res. Lett., 29 (9), 1347, doi:10.1029/2001GL014265, 2002.

A new method using energetic particles to remotely sound the high-latitude

magnetopause in three-dimensions is presented. Less than two gyro radii from

an absorbing boundary a trapped particle distribution appears to be non-

gyrotropic, as particles start to cross the boundary. If the magnetic field and

the particle mass and energy is known, it is possible to derive the direction and

distance to the magnetopause just by examining the azimuthal distribution of

locally mirroring particles. Combining observations from at least three

spacecraft gives a three-dimensional picture of the magnetopause surface. This

analysis has been performed for three of the Cluster spacecraft during a high-

latitude boundary crossing on January 14, 2001. The very first results give a

consistent overall picture of how the magnetopause boundary is coming

towards, just passing, and then retreating away from the spacecraft. This analysis clearly illustrates that the magnetopause ion sounding technique can be used to remotely study the three-dimensional orientation and location of the magnetopause surface.

Paper 3: ESR mapping of polar-cap patches in the dark cusp, H. C. Carlson, K.

Oksavik, J. Moen, A. P. van Eyken, and P. Guio, Geophys. Res. Lett., 29 (10), 1386, doi:10.1029/2001GL014087, 2002.

A new observational technique is developed taking maximum advantage of the new software and measurement capabilities of the EISCAT Svalbard radar.

Using rapid elevation or azimuth scans the full thermal and plasma properties of high-latitude ionospheric patches can be studied on time scales near 2 minutes. In this paper we present the first ever measurement of an ionospheric patch in full darkness in the noon region where patches are believed to form.

These observations present the first experimental evidence for the Lockwood- Carlson class of mechanisms for patch formation by plasma injection. The new measurement capability allows the study of a broad class of mesoscale plasma flow-transients thought to occur over time scales of 8-10 minutes. Such transients may significantly drive global convection. In the paper we demonstrate both the validity and the need for this new measurement capability, by presenting a transient flow reversal sweeping across a 500 km by 1000 km area, with initial reversal in 4 minutes, and recovery within 6 minutes.

Paper 4: Simultaneous optical, CUTLASS HF radar, and FAST spacecraft observations: Signatures of boundary layer processes in the cusp, K. Oksavik, F.

Søraas, J. Moen, R. Pfaff, J. A. Davies, and M. Lester, Submitted to Ann. Geophys., 2002.

This paper discusses counterstreaming electrons, electric field turbulence, HF radar spectral width enhancements, and field-aligned currents in the southward IMF cusp region. Electric field and particle observations from the FAST spacecraft are compared with CUTLASS Finland spectral width enhancements and ground-based optical data from Svalbard during a meridional crossing of the cusp. The observed 630 nm rayed arc (Type-1 cusp aurora) is associated with stepped cusp signatures. Simultaneous observations of counterstreaming low-energy electrons on open magnetic field lines lead us to propose that such electrons may be an important source for rayed red arcs through pitch angle scattering in collisions with the upper atmosphere. The observed particle precipitation and electric field turbulence are found to be nearly collocated with the equatorward edge of the optical cusp, in a region where CUTLASS Finland observed enhanced spectral width. The electric field turbulence extends far poleward of the optical cusp. On this basis we suggest that electric field turbulence is the cause for the wide HF spectra in the cusp region.

Furthermore, FAST encountered two narrow and highly structured field-

aligned current pairs flowing near the edges of cusp ion steps.

Future Prospects

The open/closed field line boundary has been investigated in low and high altitudes combining multi-instrument techniques. In high-altitude the dynamics of the open/closed field line boundary can be studied remotely using properties of the three- dimensional distributions of energetic ions as in Paper 2. In low-altitude data from spacecraft, ground optics, and radars (both coherent and incoherent) can be used.

Low-altitude spacecraft data can locate the open/closed field line boundary using the 30 keV electron trapping boundary or stepped cusp signatures. However, it is necessary to study the relationship between different instrumental techniques, as illustrated in Paper 1. Radars can monitor the ionospheric structure and dynamics, and with the new operational mode presented in Paper 3 the scientific capabilities of using the EISCAT Svalbard radar are greatly enhanced. Furthermore, Paper 4 presents evidence that broadband electric field turbulence and enhanced spectral width may be signatures of open field lines.

Clearly continued work within this field of research is important in order to address

and examine the results of the four papers. Ground-based optics can monitor the

auroral emissions, provided the sky is clear and the moon is down, and ground-based

instrumentation provide high temporal and spatial resolution and remove some of the

time and space ambiguity present in in-situ spacecraft data. Complemented with new

three-dimensional observations from the Cluster mission ground-based observations

will most certainly provide answers to many problems related to the solar wind –

magnetosphere – ionosphere coupling. Furthermore, the new operational mode of the

EISCAT Svalbard radar will provide great opportunities for studies of the formation

and motion of ionospheric polar cap patches that dominate the polar cap 50% of the

time when IMF is southward. During this Dr.Scient. project several observational

campaigns have been carried out with the EISCAT Svalbard radar and ground-based

optics, providing unique data sets to attack a range of still unresolved problems; (1)

the relationship between FTEs (PMAFs) and patch formation, (2) to which degree

particle impact ionization contributes to locally produced F-region plasma, (3)

whether patch formation is controlled by the interplanetary magnetic field, (4)

whether patch formation is associated with enhanced recombination/flow channel of

enhanced Joule heating, (5) whether there is a relationship between patches and

enhanced HF backscatter and if these features are related to PMAFs (candidate FTEs),

(6) how patches exit the nightside polar cap, and (7) how plasma structuring within

patches affects the accuracy of modern communication and navigation systems in the

polar cap region. It is clear that these problems can only be resolved using multi-

instrument techniques. Such high-resolution data analysis is, however, extremely time

consuming, and it is therefore desireable to continue this analysis in the years to

come.

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Paper 1

Optical and particle signatures of magnetospheric boundary layers near magnetic noon:

Satellite and ground-based observations, K. Oksavik, F. Søraas, J. Moen, and W. J. Burke, J. Geophys. Res., 105 (A12), 27555, 2000.

Copyright 2000 American Geophysical Union.

Paper 2

Three-dimensional energetic ion sounding of the magnetopause using Cluster/RAPID, K. Oksavik, T. A. Fritz, Q.-G. Zong, F. Søraas, and B. Wilken,

Geophys. Res. Lett., 29 (9), 1347, doi:10.1029/2001GL014265, 2002.

Copyright 2002 American Geophysical Union.

Three-dimensional energetic ion sounding of the magnetopause using Cluster/RAPID

K. Oksavik,^1,2T. A. Fritz,³Q.-G. Zong,³F. Søraas,¹ and B. Wilken^4,5

Received 25 October 2001; revised 4 February 2002; accepted 5 March 2002; published15May 2002.

[1] We present new results using energetic particles to remotely sound the high-latitude magnetopause in three-dimensions. Less than two gyro radii from an absorbing boundary a trapped particle distribution appears non-gyrotropic, as particles start to cross the boundary. Knowing the magnetic field and the particle mass and energy, it is possible to derive the direction and distance to the magnetopause just by examining the azimuthal distribution of locally mirroring particles. Combining observations from at least three spacecraft gives a three-dimensional picture of the magnetopause surface. We have performed this analysis for a high-latitude boundary crossing on January 14, 2001. The very first results give a consistent overall picture of how the magnetopause boundary is coming towards, just passing, and then retreating away from the spacecraft. This clearly illustrates that the magnetopause ion sounding technique can be used to remotely study the three-dimensional orientation and location of the magnetopause surface. INDEX TERMS: 2720 Magnetospheric Physics: Energetic particles, trapped; 2724 Magnetospheric Physics: Magnetopause, cusp, and boundary layers; 2731 Magnetospheric Physics: Magnetosphere—outer; 2794 Magnetospheric Physics: Instruments and techniques

1. Introduction

[2] In July and August 2000 the four Cluster spacecraft were successfully launched into orbit. The tetrahedron formation of the spacecraft provides the first real opportunities to study the physics of the magnetosphere in three dimensions as a function of time.

Previous single or dual spacecraft missions have always had a problem separating temporal and spatial variations.

[3] In this paper we present a new technique to remotely sense the magnetopause in three dimensions as a function of time. An ion moving perpendicular to a uniform magnetic field will due to the Lorentz force termqvBperform a circular motion, whereqis the ion charge,vis the particle velocity, andBis the magnetic field intensity. The ion gyro radius of this motion is given asffiffiffiffiffiffiffiffiffi p2mE

qB; where m and E are the ion mass and energy, respectively. However, if the magnetic field contains a sharp scattering boundary, the ion will not be able to perform its original circular motion. This leads to a net loss of initially trapped ions across the boundary, which will create a distinct signature in the ion distribution.

[4] Such studies use these characteristics of three-dimensional energetic particle distributions observed near the magnetopause

[Williams, 1979; Williams et al., 1979] to estimate the location, orientation and velocity of the magnetopause in a direction perpendicular to the magnetic field. Fahnenstiel [1981] used energetic > 24 keV ion distributions from the ISEE-2 spacecraft to remotely sound the ion trapping boundary and to study standing waves at the dayside magnetopause. Later, Fritz and Williams [1984] used the same technique to study the structure and topology of the subsolar magnetopause. The same method has also been employed to data from other spacecraft such as the HEP-LD instrument onboard GEOTAIL [e.g. Zong et al., 2000].

2. Instrumentation

[5] The present study uses data from three of the Cluster spacecraft (Rumba, Samba, and Tango) during one of the first days of joint operation. The imaging energetic particle spectrom- eter RAPID (Research with Adaptive Particle Imaging Detectors) measures the velocity vector and energy of both electrons and ions.

The current paper will, however, only focus on ion data. Different ion species are separated using a time-of-flight (T) vs. energy (E) telescope in front of a solid state detector. The atomic mass (A) of the detected particle is thus given asA/ET², where the energy range is from 50 to 1500 keV. The RAPID ion sensor is designed to cover 12 angular intervals over a 180field-of-view with respect to the spacecraft spin axis. Furthermore, the spin plane of the space- craft is divided into 16 sectors, giving 192 independent samples of the unit sphere in velocity space. A more detailed description of the instrument has been given by [Wilken et al., 1997, 2001].

3. The Ion Sounding Technique

[6] The energetic ion sounding technique uses the relatively large ion gyro radius for remote sensing of a scattering boun- dary like the magnetopause. On closed field lines and away from any scattering mechanisms the three-dimensional ion dis- tribution is found to be trapped, with high fluxes of locally mirroring particles and two loss cones parallel to the magnetic field direction. In the upper left panel of Figure 1 an example of such a trapped ion distribution is shown. Note the high ion flux along the solid black line marking the 90 pitch angle region. If the spacecraft is less than two ion gyro radii away from the scattering mechanism, locally mirroring ions from some direc- tions are prevented from performing a full gyration. This gives an asymmetric or non-gyrotropic distribution like in the upper middle panel of Figure 1. Significant fluxes of ions are only observed along the solid contour between f1 and f2 (to be defined below). The dotted part of the contour shows that 90 ions are absent. The longer this dotted line is, the closer the spacecraft is to the scattering mechanism. The azimuthal location of f1 and f2 gives the rotation b of the scattering boundary relative to the spacecraft coordinate system. However, if the spacecraft itself has crossed the scattering boundary, no signifi- cant locally mirroring ion fluxes will be seen at all, and one only observes a background distribution similar to the one in the upper right panel of Figure 1.

[7] To our knowledge the magnetopause ion sounding techni- GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO.9, 10.1029/2001GL014265, 2002

1Department of Physics, University of Bergen, Bergen, Norway.

2Also at University Courses on Svalbard, Longyearbyen, Norway.

3Center for Space Physics, Boston University, Boston, Massachusetts, USA.

4Max-Planck-Institut fu¨r Aeronomie, Katlenburg-Lindau, Germany.

5Deceased.

Copyright 2002 by the American Geophysical Union.

Figure 1. The first row gives three characteristic ion distributions; a trapped distribution on closed field lines, a non-gyrotropic distribution less than two ion gyro radii away from the magnetopause, and a background distribution found outside the magnetopause.

The middle row gives the distance R in ion gyro-radii from Cluster to the magnetopause and the magnetopause tilt angleb, both derived using the energetic ion sounding technique and data from the RAPID instrument. The bottom row gives the corresponding boundary velocities (assuming that our sounding ions are 60 keV protons in a 15 nT field) and the position of the Cluster spacecraft on January 14, 2001.

61

-

see e.g.Williams[1979]. The vectorRfrom the spacecraft to the magnetopause is given by

R¼Rsinð ÞXb ⁰þRcosð ÞYb ⁰

whereRis the distance from the spacecraft to the magnetopause.

At low latitudes X⁰ XGSE and Y⁰ YGSE, since B is approximately parallel to the ZGSE axis. However, the data set we present in this paper is from high-latitudes, which will further complicate our geometry. The offset between theZGSEaxis and the magnetic field vector must be taken into account, and we define a new coordinate systemX⁰Y⁰Z⁰orX⁰Y⁰B, since theZ⁰axis is chosen to be parallel to the magnetic field. Furthermore, we define theX⁰ axis to be perpendicular to B, located in the XZGSE plane, and rotated an angleqXrelative to theXGSEaxis. Finally, theY⁰axis is chosen so that the X⁰Y⁰B system is orthogonal. According to Figure 2 the transformation is then given by:

X⁰¼cosð ÞXqX GSEsinð ÞZqX GSE

Y⁰¼B

B X⁰¼ B_Y

B sinð ÞXqX GSE

þ B_Z

B cosð Þ þqX B_X B sinð ÞqX

YGSE B_Y

B cosð ÞZqX GSE

whereqX= tan¹(BX/ |BZ| ).

[8] The magnetopause tilt angle bis found from the azimu- thal distribution of the ions, by reading out the anglesf1andf2

of where the significant fluxes start and end, respectively (see Figure 1). Both angles are measured clockwise from the the X⁰ axis, and are calculated in the following way:

f1¼C₁=C_T 360þ60:167

f2¼C₂=CT 360þ60:167

where C1 and C2 are the positions of f1 and f2, respectively, measured along the 90contour, andCTis the total length of this contour. The last term adjusts for an offset relative to Sun-Earth line of the first azimuth sector of the RAPID instrument. From

Figure 3 symmetry (f1 90 b= 270 f2 +b) gives the magnetopause tilt angle relative to the Y’ axis:

b¼ðf1þf2Þ=2180

The distance from the spacecraft to the magnetopause, R, is expressed by the gyro-radiusr(see Figure 3) and is given as:

R¼rrsinð Þd where

d¼90þðf1f2Þ=2

4. January 14, 2001

[9] January 14, 2001, was one of the first days when the Cluster spacecraft were in full operation. Several papers have been published from this date, believed to be the first real Cluster magnetospheric cusp encounter.Opgennorth et al. [2001] give a nice review of the large scale geophysical context (solar wind, Cluster observations, low-altitude spacecraft data, and ground- based instrumentation).Moen et al. [2001] present ground-based all-sky images from Svalbard. Zong et al. [2001] provide an overview of the RAPID observations. In this paper we would like to go a step further and study the dynamics of the magnetopause during this boundary layer crossing, as deduced from energetic ion observations from the RAPID instrument.

[10] The two middle panels of Figure 1 give the magnetopause distanceRin ion gyro-radii and the magnetopause tilt anglebfor the three Cluster spacecraft Rumba, Samba, and Tango. To indicate whether the spacecraft is inside or outside the magnetopause, we have setR= 0 when the spacecraft is outside andR= 2 when the spacecraft is more than two ion gyro-radii inside the absorbing boundary assumed to be the magnetopause. The magnetopause is seen to be in constant motion, as seen by all three spacecraft. The overall magnetopause distances and the fact that the magnetopause is observed duskward of all spacecraft, are consistent with the Figure 2. Geometry near the high-latitude magnetopause,

showing the transformation from the localX⁰Y⁰B⁰magnetic field

Figure 3. Geometry near the magnetopause, showing how the non-gyrotropic ion distribution from a single spacecraft (upper middle panel of Figure 1) can be used together with the ion gyroradius to find the distance R to the magnetopause and the magnetopause tilt angleb.

OKSAVIK ET AL.: MAGNETOPAUSE ION SOUNDING 61

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