STUDIES OF AURORAL FIELD-ALIGNED CURRENTS WITH MAGSAT

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1 THOMAS A. POTEMRA STUDIES OF AURORAL FIELD-ALIGNED CURRENTS WITH MAGSAT Electric currents that flw between the utermst bundaries f the magnetsphere and the aurral znes, smetimes deliver mre pwer t the earth than is generated in the entire United States at any given time. These field-aligned "Birkeland" currents are detected n a regular basis by Magsat. INTRODUCTION Fllwing earlier suggestins by Edmund Halley and Anders Celsius that the aurral phenmena behave magnetically, Kristian Birkeland discvered in his plar expeditins f that large-scale currents were assciated with the aurra. He was als the first t suggest that these currents riginated far frm the earth and that they flwed int and away frm the plar atmsphere alng the gemagnetic field lines. The existence f such fieldaligned currents was widely disputed because it was nt pssible frm a study f nly surface magnetic field measurements t distinguish unambiguusly between current systems that are field aligned I and thse that are cmpletely inspheric. 2 Furthermre, theretical calculatins 3,4 predicted that the effects f field-aligned currents may be impssible t detect with grund-based magnetmeters fr certain mdels f these current systems. The first experimental cnfirmatin f the presence f field-aligned ("Birkeland") currents was prvided by A. J. Zmuda and his clleagues 5,6 wh studied magnetic disturbances in the plar regins f the earth, bserved with the single-axis magnetmeter used in the attitude determinatin system n the APL/ U.S. Navy 5E-I (l963-38c) satellite. That satellite, launched n September 28, 1963, int a nearly circular plar rbit at an altitude f 1100 km, carried an array f particle detectrs and a fluxgate magnetmeter. Zmuda 5,6 discvered that disturbances nrmal t the main gemagnetic field were ften bserved with 5E-l in the same regin f the earth assciated with a variety f aurral phenmena. It was sn realized that the transverse disturbances culd be interpreted as being caused by electric currents flwing alng the gemagnetic field lines, and thus the first study f the glbal characteristics f Birkeland currents was begun at APL. Recgnizing the imprtance f sensitive satellite magnetic field measurements t the study f Birkeland currents, Armstrng and Zmuda 8 suggested a 228 mdificatin t the attitude magnetmeter system fr the APL/ U.S. Navy Triad satellite. The Triad satellite was launched int a plar rbit at an altitude f 800 km n September 2, 1972, and, until the launch f Magsat n Octber 30, 1979, was the nly satellite making cntinuus and highreslutin magnetic bservatins in the plar regins. The Triad magnetmeter prvides 2.25 vectr samples per secnd with a reslutin f 12 nanteslas (l nt = 10-9 weber per square meter), made pssible by the 13-bit analg-t-digital cnverter incrprated int the system fllwing the suggestin f Armstrng and Zmuda. The Triad magnetic field bservatins were used t determine fr the first time the flw directins, spatial distributin, and intensities f Birkeland currents and their relatinships t a variety f gephysical phenmena such as ptical emissins, currentdriven plasma instabilities, inspheric currents, gemagnetic strms, and interplanetary phenmena. During its brief lifetime, the imprved Magsat magnetic field bservatins were crrelated with the Triad bservatins t cmpile mre infrmatin abut the imprtant BirkelaI)d currents and t calibrate the Triad magnetmeter. Frtunately fr these studies, Magsat was launched near the time f the peak slar cycle (the largest in many II-year cycles) s that disturbances prduced by the Birkeland currents were bserved n almst every rbit. The imprtance f Birkeland currents t the cupling between uter space and the aurral atmsphere is emphasized by their ttal intensity, which ranges between 10 6 and 10 7 A, and by the energy they dissipate in the upper atmsphere, which can exceed by a cnsiderable factr the energy depsitin assciated with the dramatic, visual, aurral frms. The Birkeland currents are als the critical ingredient in a variety f cmplicated plasma phenmena that have imprtant applicatins t earth (assciated with substrms and the aurra), t Jupiter (assciated with 10- related radi emissins),9.10 and t the galaxy (related t cmets and duble radi surces). II Jhns Hupkins A PL Technical Diges{

MAGNETIC DISTURBANCES AND FIELD-ALIGNED CURRENTS The magnitude and directin f equivalent currents assciated with magnetic disturbances, ~B, are determined frm the relatinship f = (II J,t) curl ~ii.

2 MAGNETIC DISTURBANCES AND FIELD-ALIGNED CURRENTS The magnitude and directin f equivalent currents assciated with magnetic disturbances, ~B, are determined frm the relatinship f = (II J,t) curl ~ii. The disturbances ccur predminantly in the east-west (gemagnetic) directin s that J = (l/j,t)(alax) My, where J is the density f the field-aligned current, x is psitive in the nrthward directin, and My is the bserved disturbance in the east-west directin. 12, 13 In this case, the frmula is the same as that fr the transverse magnetic disturbance due t current sheets f infinite lngitudinal extent. At the altitude f Triad, a change in By equal t 100 nt ver 1 0 f latitude is equivalent t a field-aligned current density f 0.64 J,tAlm 2 The magnitude f the current intensity, Vdx, is given directly by the amplitude f the magnetic disturbance My by Vdx = (1/ J,t) My. A 100 nt disturbance crrespnds t a current intensity f 0.08 A i m. An example f a transverse magnetic disturbance ften bserved with Triad is shwn in Fig. 1 (frm Ref. 13) with the deduced field-aligned current density and intensity. PRINCIPAL CHARACTERISTICS OF BIRKELAND CURRENTS Large-scale Birkeland currents are cncentrated in tw principal areas that encircle the gemagnetic ple Figure 2 is a summary f the average spatial distributin in the nrthern high-latitude regin determined frm 493 Triad passes during weakly disturbed gemagnetic cnditins and 366 Triad passes during an active perid (frm Ref. 16.) The Birkeland current flw regins have been arbitrarily designated by Iijima and Ptemra 14, 15 as +-' C C1l C c. E u "OJ:: ~5 ~ 0 C1l >- CIl <l 65 Inspheric electric f ield (---suthward) Current ut f Current int tftht!rrn! 1540 I nvariant latitude, 1\ (deg) UT May 4, 1973 J II = 5 x 10-7 A/ m2 III = 8 x 10-2 Ai m Fig. 1-An example f a magnetic disturbance transverse t the gemagnetic field measured by Triad n May 4, 1973 (frm Zmuda and Armstrng 13 ). The density and flw directin f the field-aligned currents prducing these disturbances are determined frm Maxwell's equatin J = (1/IL) curl LlB in the manner described in the text. The directin f the apprpriate electric field, als shwn, can prduce a suthward current flwing hrizntally in the insphere t cuple the upward and dwnward flwing field-aligned currents. V lume I, Number 3, (a) C=:J Current int insphere C=:J Current away frm insphere \\-+---l6 Fig. 2-Statistical distributins f the lcatin and flw directins f large-scale Birkeland currents during weakly disturbed gemagnetic cnditins (a) and during disturbed perids (b) (frm Ref. 16). Regin 1 lcated at the pleward side and Regin 2 lcated at the equatrward side. The Regin 1 Birkeland currents flw int the insphere in the mrning sectr and away frm the insphere in the evening sectr, whereas the Regin 2 currents flw in the ppsite directin at any given lcal time. The basic flw pattern f Birkeland currents remains unchanged during gemagnetically inactive and active perids (Fig. 2), althugh the regins widen and shift t lwer latitudes during disturbed perids (similar t ptical aurral frms by Feldstein 17 ). Figure 3 (frm Ref. 18) is a schematic diagram f the large-scale Birkeland currents that flw int and away frm the aurral regins shwn in Fig. 2 and frm cne-shaped regins. The current flw reverses directin near midnight and near nn. This current system is permanent, with the ttal amplitude ranging between 10 6 and 10 7 A, depending n the slar-terrestrial cnditins. The striking visual frms f the aurra ccur nly when the incming particles are energetic enugh t penetrate t lw altitudes and excite the atmspheric gas. Analyses f Triad magnetmeter data acquired ver the suth plar regins and recrded at McMurd, Antarctica, indicate that the basic flw pattern shwn in Fig. 2 fr the nrth ple is the same fr the suth ple. 19 At any given lcal time and invariant latitude lcatin, the field-aligned current flw is the same. The average values f field-aligned current density and intensity in the varius regins and sectrs discussed abve are listed in Table 1 (frm Ref. 16). The ttal current, estimated by integrating the bserved current intensity ver the lcal time extent f the apprpriate regin, is als listed in Table 1. The ttal amunt f current flwing int the insphere (including Regins 1 and 2) is always equal t the ttal current flwing away (within 51tJ) fr every level f magnetic activity. The fact that current cntinuity is always preserved is imprtant t (b) 229

Fig. 3-Cne shaped regins frmed by Birkeland currents flwing int and away frm the aurral regin. The currents n a given sheet reverse directin near midnight and nn (frm Ref. 18).

3 Fig. 3-Cne shaped regins frmed by Birkeland currents flwing int and away frm the aurral regin. The currents n a given sheet reverse directin near midnight and nn (frm Ref. 18). Table 1 A VERAGE CHARACTERISTICS OF LARGE-SCALE FIELD-ALIGNED CURRENTS Afternn t Midnight Parameter Regin 1 Regin 2 Regin 1 Current density (/LA / m 2) Quiet Disturbed Ttal current (10 6 A) Quiet Disturbed Midnight t Frenn A psitive value dentes current flw int the insphere. A negative value dentes flw away frm the insphere. Values are averages ver specified regins. Regin the develpment f the mdels f the three-dimensinal current system that cuples the magnetsphere with the lwer atmsphere and insphere. It is als significant that the ttal field-aligned current in Regin 1 is significantly larger than the Regin 2 ttal current during relatively quiet cnditins, while during active perids the ttal current flw in bth regins becmes nearly equal. 230 MAGSAT OBSERVATIONS OF BIRKELAND CURRENTS The Magsat spacecraft was launched int a dawn-dusk synchrnus rbit, whereas Triad is in a plar rbit and appears t precess westward with respect t the earth-sun line at a rate f apprximately a degree per day. The rbital planes f the tw spacecraft were nearly cincident in Nvember 1979, when magnetic field measurements were made at nearly the same pint in time and space (separated nly by their altitude difference and the time differential related t their rbital perids - a value that cannt exceed ne-half the rbit perid, r apprximately 50 minutes, when they are mving in the same directin). Data were acquired by Magsat and Triad during a disturbed perid n Nvember 7, 1979 at nearly the same place in the nrth aurral regin. Segments f their rbital tracks are shwn in Fig. 4 superimpsed n the statistical distributin f Birkeland currents shwn in Fig. 2 determined fr disturbed cnditins. The Triad bservatins were made at 17: 12:30 UT, and the Magsat bservatins 25.5 minutes later (at 17:38 UT). Regins f upward- and dwnwardflwing Birkeland currents detected by Magsat and Triad are shwn by the shaded areas f Fig. 4. They are cincident and are clse t the statistical distributin. The rientatins f the transverse Jhns Hpkins A PL Technical Digest

4 magnetmeter axes (dented A and B) fr the tw spacecraft with respect t their velcity vectrs are shwn in the upper right crner f Fig. 4. The A and B axes fr Triad are directed 135 and 45 0, respectively, frm the velcity vectr, and fr Magsat are parallel and nrmal, respectively, t the velci ty vectr. T riad/ Magsat Nv. 7, 1979 (17:12:30 UT/ 17 :38:00 UT) IALI > 100 nt 12 18~--+r-r----~----~ ~~~~6 c::::j Currents int insphere [=:::J Currents away frm insphere Fig. 4-0rbital tracks f Triad and Magsat superimpsed n the statistical pattern f Birkeland currents shwn in Fig. 2. The lcatins and flw directins f the Birkeland currents detected by the tw satellites are shwn by the shaded areas. Theretical magnetic field values (cmputed frm the 1965 Mdel Internatinal Gemagnetic Reference Field) extraplated t the perid f bservatin were subtracted frm the Triad and Magsat bservatins. The resulting disturbance vectrs were rtated frm the spacecraft crdinate systems t the gemagnetic reference frame. The cmpnents in the gemagnetic east-west directin (i.e., tangent t the circles in Figs. 2 and 4) frm Triad and Magsat are shwn in Fig. 5. These preliminary data shw the amazing similarity f the details in the transverse magnetic disturbances measured by different spacecraft. The disturbances are ver 1000 nt and can be interpreted as resulting frm a pair f intense Birkeland currents flwing int the insphere between abut 67 and 65 0 magnetic latitude and away frm the insphere between abut 65 and 61 0 magnetic latitude. The density f these currents is abut 2 J-tA/m 2 and their intensity is abut 0.8 Aim. The prcessing f the Magsat data is in a preliminary stage and wrk is nly beginning n the investigatin f disturbances assciated with Birkeland currents. The spatially separated measurements prvided by Magsat and Triad will permit the determinatin f the spatial extent f the Birkeland currents and f the differences, if any, in the intensities f these currents ver sunlit and dark prtins f the plar insphere. The imprved sensitivity f the Magsat magnetmeter and its lwer altitude (in cmparisn t Triad) will als prvide data n the ptential field cntributin f inspheric aurral currents that flw at an altitude f abut 100 km (the magnetic effects f which have been bserved n the grund n a regular basis since Birkeland's riginal wrk). The ultimate bjective f these studies is t understand and define the variable high-latitude magnetic disturbances t assist in the Magsat prgram's gal f btaining an accurate, up-t-date quantitative descriptin f the earth's main gemagnetic field. UT ML T 0426 MLA l e: iii" <I Ql U lij 0.0 ~ :c u.~ Ql e: OJ E ~----~----~----~----~----~----~~ Triad/Chatanika 11 /7/79 17 :09 (UT) / Magsat 11 /7/79 17 :25 (UT)...-~. :!, : <J.,.,.:,' ~: :.. j :i~ ~.. i!\:j., \l'~. UT - Universal time (s) ML T - Magnetic lcal time (h, min) MLA - Magnetic latitude (deg) -1000~U-T ~ ~ L ~ ~ ~1LO ~ ~2-20--~ M L T M LA Vlume 1, Number 3,1980 Fig. 5-The gemagnetic east west cmpnent determined frm the Magsat and Triad bservatins. The hrizntal scale lists universal time (UT) in secnds, magnetic lcal time (ML T) in hurs and minutes, and magnetic latitude (MLA) in degrees. 231

5 REFERENCES and NOTES IK. Birkeland, The Nrwegian Aurra Plaris Expeditin, , Aschehug, Osl, rway (1908). 2E. H. Vestine and S. Chapman, " The Electric Current System f Gemagnetic Disturbances," Terr. Magn. AliI/s. Electr. 43, p. 351 (1938). 3N. Fukushima, "Equivalence in Grund Gemagnetic Effect f Chapman-Vestine's and Birkeland-Alfven's Electric Current Sys tems fr Plar Magnetic Strms," Rep. /ns. Space Res. Jpn. 23, p. 219 (1969). 4N. Fukushima, " Generalized Therem fr N Grund Magnetic Effect f Vertical Current Cnnected with Pedersen Currents in the Unifrm Cnductivity Insphere," Rep. /ns. Space Res. Jpn. 30, p. 35 (1976). 5A. J. Zmuda, J. H. Martin, and F. T. Heuring, " Transverse Magnetic Disturbances at 1100 km in the Aurral Regin," 1. Gephys. Res. 71, p (1966). 6A. J. Zmuda, F. T. Heuring, and J. H. Martin, "Day Side Magnetic Disturbance at 1100 km in the Aurral Oval," J. Gephys. Res. 72, p.1115(1967). 7The 5E series f satellites was designed by APL in the perid t make scientific and engineering measurements f the envirnment f the Transit navigatin satellites. Fur were launched piggyback n Transit Type 5 satellites, and three achieved rbit. Studies f the 5E spacecraft data were carried ut by scientists arund the wrld, and 43 articles were event ually published n their findings in prfessinal jurnals. The 5E-I satellite became ne f the mst prductive ever launched, still prducing data in its eleventh year (a full slar cycle), when a sympsium was held at APL n September 27, 1974, celebrating the utstanding success f the 5E prgram. 8 J. C. Armstrng and A. J. Zmuda, "Triaxial Magnetic Measurements f Field-Aligned Currents at 800 km in the Aurral Regin: Initial Results," 1. Gephys. Res. 78, p (1973). 9p. A. Clutier, R. E. Daniell, Jr., A. J. Dessler, and T. W. Hill, "A Cmetary Insphere Mdel fr 10," Astrphys. Space Sci. 55, p. 93 (1978). IDA. J. Dess ler and T. W. Hill, "Jvian Lngitudinal Cntrl f 10- Related Radi Emissins," Astrphys. J. 227, p. 664 (1978). II H. Alfven, "Electric Currents in Csmic Plasmas," Rev. Gephys. SpacePhys. IS, p. 271 (1977). 12A. J. Zmuda and J. C. Armstrng, "The Diurnal Variatin f the Regin with Vectr Magnetic Field Changes Assciated with Field Aligned Currents," 1. Gephys. Res. 79, p (1974). 13 A. J. Zmuda and J. C. Armstrng, "The Diurnal Flw Pallern f Field-Aligned Currents," J. Gephys. Res. 79, p (1974). 14T. lijima and T. A. Ptemra, " The Amplitude Distributin f Field Aligned Currents at Nrthern High Latitudes Observed by TRIAD," J. Gephys. Res. 81, p (1976). 15T. lijima and T. A. Ptemra, "Field-Ali gned Currents in the Dayside Cusp Observed by TRIAD," J. Gephys. Res. 81, p (1976). 16T. lijima and T. A. Ptemra, "Large-Scale Characteristics f Field Aligned Currents Assciated with Substrms," J. Gephys. Res. 83, p. 599 (1978). l7y. I. Feldstein, "Peculiarities in the Aurral Distributin and Magnetic Disturbance Distributin in High Latitudes Caused by the Asymmetrical Frm f the Magnetsphere," Planet. Space Sci. 14, p.121 (1966). 18T. A. Ptemra, "Aurra Brealis: The Greatest Light Shw n Earth," Smithsnian 7, p. 64 (1977). 19T. A. Ptemra, S. Favin, and T. lijima, "Field-Aligned Currents in the Nrth and Suth Aurral Regins Measured with TRIAD," EOS, 56, p. 617 (1975). 232 Jhns Hpkins APL Technical Digest

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