Global auroral response to negative pressure impulses

K. Liou,1 P. T. Newell,1 T. Sotirelis,1 and C.-I. Meng1

Received 1 February 2006; revised 24 March 2006; accepted 24 April 2006; published 3 June 2006.

[1] It is well known that sharp increases/decreases in thesolar wind dynamic pressure can result in suddencompression/decompression of the magnetosphere andsubsequent magnetic positive/negative impulses (SI+/SI�)detected on the ground magnetometers. While the large-scaleenhancement of aurora during an SI+ has been wellestablished, the response of aurora to an SI� is still littleknown. This prompts an interesting question whether theresponse of the global aurora to an SI�mirrors the response toan SI+. This letter reports results from a study of auroralimages, acquired from the ultraviolet imager (UVI) on boardthe Polar satellite, during 13 SI� events. It is found that, inmost cases, the luminosity of the aurora indeed showed aclear decrease almost immediately after the decompression.In some cases, the luminosity decrease exhibits a day-to-night fading effect and is consistent with the tailwardpropagation of the magnetosphere decompression front.Auroral particle observations from DMSP indicate thatreduction of CPS electron precipitation is the major causeof the large-scale auroral dimming. We propose that aninduction electric field triggered by the sudden expansion ofthe magnetosphere at the expansion front along withadiabatic decompression and magnetic reconfiguration areresponsible for the observed effect. Citation: Liou, K., P. T.

Newell, T. Sotirelis, and C.-I. Meng (2006), Global auroral

response to negative pressure impulses, Geophys. Res. Lett., 33,

L11103, doi:10.1029/2006GL025933.

1. Introduction

[2] The solar wind dynamic pressure plays the major rolein controlling the overall size and shape of the magneto-sphere. When a fast interplanetary shock or a suddenincrease in the solar wind dynamic pressure makes a contactwith the Earth’s magnetosphere, the magnetosphere is com-pressed. In response to the compression, the Chapman-Ferraro current becomes enhanced and produces a suddencommencement (SC) or positive sudden impulse (SI+) on theground magnetometers. On the other hand, when a reverseshock or a sudden decrease in the solar wind dynamicpressure (‘‘negative pressure pulses’’) arrives at the Earth,the magnetosphere becomes decompressed and a negativesudden impulse (SI�) is detected by the ground magneto-meters [Araki, 1994].[3] Interplanetary shocks have been attributed to optical

auroral enhancements seen on the ground [Vorobyev, 1974]and in space [Craven et al., 1986; Spann et al., 1998].Sudden compression of the magnetosphere by shock impact

results in sudden brightening of aurora in the dayside ovalthat propagates antisunward [Zhou and Tsurutani, 1999]and in the dayside subauroral regions moving equatorward[Liou et al., 2002; Zhang et al., 2002]. It is generallybelieved that enhanced pitch angle scattering of pre-existingmagnetosphere particles into the loss cone is responsible forthe enhanced aurora luminosity. On the other hand, studiesof aurora associated with SI�s are rare. An importantquestion concerning the solar wind pressure effect is wheth-er or not the response of the global aurora to SI�s is just theopposite to that of SI+s. There appears to be only one reporttouching on the subject [Sato et al., 2001]. Based onground-based all-sky camera images, they found enhance-ments of aurora in the afternoon sector of the southern ovalafter an SI�. The result of Sato et al. [2001] seems tosuggest that any type of disturbance creates more pitchangle scattering and more aurora. Because of the limitedfield of view imposed on their observations, the response ofthe aurora to SI�s cannot be addressed on a global scale. Itis likely that depending on the location and the physicalmechanism involved the effect of an SI� could be toincrease or decrease aurora. The purpose of this study isto examine this global response.

2. Observations

[4] This study starts with a list of 28 negative suddenimpulses (SI�s) identified by Takeuchi et al. [2002] fromthe mid-latitude Sym-H index [Iyemori and Rao, 1996].These SI� events were associated with a sharp decrease inthe solar wind dynamic pressure originating from a varietyof solar wind plasma sources such as corotating interactionregions, magnetic clouds, heliospheric plasma sheet, andplasma holes. After examining auroral image data from theultraviolet imager (UVI) [Torr et al., 1995] on board thePolar satellite, we found 13 concurrent events. These eventsare listed in Table 1.[5] Polar UVI images are then used to determine changes

of auroral luminosity associated with the 13 SI� events.Specifically, we use UVI images of Lyman-Birge-Hopfieldlong (LBHl) band (160–180 nm) to measure the luminositychanges because auroral emissions in this band are approx-imately proportional to the energy flux of precipitatingelectrons [Strickland et al., 1983]. It is found that in mostevents the auroral luminosity showed clear decreases afterthe SI� onset. The last event, 3 December 1999, in the listwill be used to demonstrate the effect.

2.1. Case Event: 3 December 1999

[6] This event is associated with the heliospheric plasmasheet (HPS). The sharp decrease in the solar wind dynamicpressure was observed by ACE (XGSM � 222 RE) during thecrossing of the HPS trailing edge at �1008 UT. The solarwind plasma and IMF parameters for the 1000–1300 UT

GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L11103, doi:10.1029/2006GL025933, 2006

1Johns Hopkins University Applied Physics Laboratory, Laurel,Maryland, USA.

Copyright 2006 by the American Geophysical Union.0094-8276/06/2006GL025933

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period are plotted in Figures 1a–1c. The sudden pressuredecrease induced a large SI� (DH � 26 nT) at 1114 UT (seeFigure 1e). The z-component of IMF was fluctuating butremained mostly negative, whereas the y-component of IMFwas positive with a sharp increase at the discontinuity.[7] A sequence of auroral images in the LBHl band from

UVI between �1100 and 1128 UT with a time separation of�2 min are shown in Figure 1f. During the negative impulse(�1114 UT), the field of view of the Polar UVI was off thecenter of the oval toward the postmidnight sector, leavingthe postnoon oval outside the field of view. Auroras wereactive in the postmidnight sector before the SI�. One minute

after the SI� onset, the luminosity of the auroras showed asignificant reduction (the first image at the third row) andthe reduction lasted more than 15 min. The reduction of theaurora occurred first on the dayside and then nightside. Theeastern end of the aurorally intensified region in the prenoon-to-dawn section shows a clear antisunward retreat. To quan-tify the auroral activity in response to the SI�, we haveintegrated auroral power over the 21–09 MLT sector above60� MLAT and the result is shown in Figure 1d. The auroralpower showed a 60% decrease (from �100 gigawatts at�1114 UT, the SI� onset time, to �40 gigawatts at�1120 UT, the end of the SI� time), and is wellcorrelated with the Sym-H index.

2.2. General Results

[8] We have carefully examined the 13 events of negativeimpulses and the result is summarized in Table 1. Duringthese impulse events, the auroral display may show com-plex variations; new aurora may appeared in some part,mostly nightside, of the oval but the overall auroral lumi-nosity show noticeable decreases for all events, except theone event on 1 August 1998, within a few minutes of theSI� onset. Nightside auroral enhancements, probably asso-ciated with substorms, occurred in a few events within�10–20 min of the SI� onset. In such cases, the nightsideauroral luminosity may increase but dimming of the auroraat some places, especially in the day oval, was still discern-ible. Percentage changes in auroral power inferred fromUVI images is listed in the last column of Table 1. No

Table 1. Negative Impulse Events [From Takeuchi et al., 2002]a

Date,yymmdd

UT,hhmm

DH,nT

IMF Bz

Before/After, nTAuroral Power% Change

961224 1654 �23 3.6/4.8 �45970627 0933 �23 4.0/�9.3 �13980108 2326 �22 9.0/�7.7 �13980423 2300 �43 11.2/11.6 �53980504 1202 �36 2.6/2.2 �32980626 1033 �40 1.6/3.5 0980801 1604 �26 �1.0/�0.3 25990106 1419 �30 �5.4/0.6 �46990211 1959 �25 21.0/24.6 �35990416 2259 �35 �7.7/�7.5 �8990518 0406 �27 14.2/22.2 �28990926 2028 �40 4.5/�9.7 �33991203 1114 �26 �3.3/�1.7 �60aIMF Bz is 10 min average.

Figure 1. (a) The solar wind dynamic pressure, (b) y- and (c) z-components of the interplanetary magnetic field (IMF),(d) auroral power (21-09 MLT sector) and (e) the Sym-H index for 10–13 UT on 03 December 1999. The solar wind andIMF data were acquired by Wind spacecraft and have been propagated to the Earth. The vertical dashed line marks the onsettime (1114 UT) of the negative impulse (SI�). (f) A sequence of false-color Northern Hemispheric auroral imagesfrom Polar ultraviolet imager (UVI) �14 min before and after the SI�. Note that the auroral power in Figure 1d ends at�1140 UT because of a sharp change in the UVI viewing angle.

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correlation between the amplitude of SI�s and auroral powerwas observed. Note that the auroral power was computedover viewable oval area, which varies from events to events;therefore, only the percentage change is provided.[9] In some of the events studied the reduction of auroral

luminosity can be seen first on the dayside and thennightside. This fading effect can be demonstrated best inmagnetic local-time keogram [Meng and Liou, 2002] shownin Figure 2. In this event, the dawnside aurora started fadingfrom 1000 MLT at �1116 UT, a couple of minutes after theSI� onset, to 0100 MLT at �1125 UT. Note that theafternoon oval was not imaged during this time. The smalltime delay may be due to the timing uncertainty of the SI�

onset. Note that not all events showed such a day-to-nightfading effect. After examining those events with the day-to-night fading feature, we found the typical time scale is �10minutes, which translates to �8 km/s in the ionosphere(assuming 70� oval latitude). Therefore the day-to-nightauroral fading is consistent with the tailward propagation ofmagnetosphere decompression front.[10] Ultimately the quench of aurora is associated with a

reduction of precipitating particle (probably mostly elec-tron) energy fluxes. To understand possible causes of thedecrease of precipitating particles, it is useful to study thecharacteristics of particle precipitation. We have searchedDMSP particle data for the 13 events and found a fewevents (06 January 1999, 11 February 1999, and 16 April1999) in which two DMSP satellites made a consecutiveoval crossing along a similar orbit (within 2 hours of MLT)during the SI� onset. Here we show the 6 January 1999event in Figure 3. In this fortuitous event, both F12 and F14satellites flew over the southern oval in the mid-morningsector along �08:30 (±00:20) MLT within 20 minutes fromeach other. The onset of SI� occurred at �14:19 UT, whichis �10 min after the F12 crossing but �10 min before theF14 crossing. Therefore, Figures 3a and 3b representprecipitating particle spectra before and after the impact ofthe negative impulse. A significant difference between thetwo spectrograms is the dramatic reductions in both theenergy flux and average energy of the structureless precip-itating electrons in the low-latitude part of the oval, perhapsassociated with the dayside extension of the plasma sheet.

For example, between 76� and 77� MLAT on the first pass(BPS), electron precipitation averaged about 0.5 ergs/cm2-s,whereas over the second pass, the oval in this same regionaveraged about 0.2 ergs/cm2-s. Indeed, it is the electron inthe keV energy range and above which are reduced. Unfor-tunately auroral images were not available during this timeperiod. The similar feature was observed in the other eventseven though the two DMSP satellites (F11 and F13) wereseparated by up to �2 hours of MLT.

3. Discussion

[11] The thirteen events that we have studied provideenough evidence of ‘‘rapid’’ drops in the overall auroralluminosity after the arrival of negative interplanetary pres-sure pulses at the Earth. The effect is opposite to thepositive pressure pulses, which enhance the auroral lumi-nosity [e.g., Craven et al., 1986; Spann et al., 1998; Zhouand Tsurutani, 1999]. However, a day-to-night propagation

Figure 2. Magnetic local-time keogram derived fromPolar UVI images in the LBHl band for the 3 December1999 SI� event.

Figure 3. Particle energy spectrograms from the (a) DMSPF12 and (b) F14 southern oval crossings during the6 January 1999 negative impulse event. The SI� onset timeis 1419 UT.

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of the effect is seen in both cases, indicating a tailwardmoving magnetospheric source associated with the tailwardmotion of the newly compressed/decompressed regions.The time scale for the reduction of the aurora is �10minutes, which is comparable to the duration of the SI�.Therefore, the oval regions that show auroral luminosityvariations must map to the interaction regions between thenegative impulses and the magnetosphere.[12] Recently, Sato et al. [2001] reported ground obser-

vations of discrete aurora in the afternoon sector during asolar wind negative pressure impulse event. They concludedthat the discrete aurora was associated with field-alignedcurrent enhancements triggered by the pressure impulse.Our observations do not necessarily disagree with theirresults because such a small scale aurora may not bedetectable by UVI because of its relatively low spatialresolution and sensitivity. On close examination of theUVI images, it is found that enhancements of small-scaledayside aurora did occur in a few events studied (e.g.,4 May 1998, 26 June 1998, and 11 February 1999), whilethe overall auroral luminosity decreased. Furthermore,unlike the discrete auroral feature observed by Sato et al.[2001], particle spectrograms from a few DMSP crossingsindicate that the gross reduction of the auroral luminosityobserved by UVI was associated with the reduction of thediffuse electron precipitation originating from the plasmasheet. Therefore, different mechanisms must have actedon different regions of the magnetosphere and producedifferent auroral displays in the ionosphere.[13] One possible explanation for the rapid quench of

aurora by negative impulses is the adiabatic expansion ofthe magnetosphere. During the impact of a negative pres-sure impulse on the magnetosphere, the outer magneto-sphere expands outward. Conservation of the first adiabaticwould decrease the perpendicular-to-parallel ratio of particleenergy and inhibit the growth of loss-cone instability[Johnstone et al., 1993]. This in turn reduces the amountof particles that may be scattered into the loss cone bywave-particle interactions. This is opposite to the adiabaticcompression mechanism proposed by Zhou and Tsurutani[1999] to explain shock-triggered aurora. However, Sato etal. [2001] argued that such a theory would predict noauroral enhancements when applying to negative pressureimpulses and is in contradiction with their findings. Sucha contradiction can be resolved if one can differentiatediscrete aurora from the background diffuse aurora. Basedon DMSP data presented in Zhou et al. [2003] and thepresent study, variations of the overall auroral luminosityassociated with changes in solar wind dynamic pressure arecontrolled mainly by the appearance/disappearance of non-structured electron precipitation. On the other hand, thesmaller scale auroral intensification reported by Sato et al.[2001] during negative impulses was associated withinverted-V electrons, and therefore, must be associated withdifferent mechanisms.[14] Another possible explanation for the reduction of

aurora associated with negative impulses is due to thereduction of the mirror ratio. It is expected that the effectof magnetospheric decompression caused by the impact of anegative impulse is strongest at the equatorial plane. Alarger decrease in the magnetic field at the equatorial planethan in the mirror points would decrease the mirror ratio

hence reduce the amount of pre-existing particles precipi-tation. However, such an effect may be compensated by thereduction of pitch angle of the plasma due to the firstadiabatic invariant.[15] While the adiabatic expansion mechanism and re-

duction of the mirror ratio may explain the reduction ofparticle precipitation, they cannot explain the decrease inthe average energy of precipitating electrons. A possiblecause is the combination of an induction electric field andthe second (longitudinal) invariant or ‘‘Fermi acceleration.’’Sudden expansion of the magnetosphere can produce aninduction electric field due to the temporal change in themagnetic field. The induction electric field in the magneto-sphere equatorial plane associated with the magnetosphericexpansion is duskward. An E � B drift could move particlessunward to larger L-shells. Because the bounce time for1-keV electrons is only a few seconds in the dayside auroralzone, which is much smaller than the magnetosphereresponse time (several minutes), the second adiabaticinvariant must be conserved. Therefore, the parallel energyof precipitating electrons will be reduced because of longermagnetic field lines. Such an explanation may also explainthe gaining of precipitating electron energy duringmagnetospheric compression reported by Zhou et al.[2003]. To test these ideas, one needs to first quantifythe compression/decompression effect on the auroralparticle energy flux and average energy with much expandedevents. Such work is out of the scope of the present paperand will be performed in the future.

4. Conclusions

[16] We have studied the response of the aurora tonegative pressure pulses (SI�s), an interesting phenomenonthat explores a complementary aspect of the previouslyestablished effects of (positive) pressure pulses in the solarwind. Based on Polar UVI images acquired during 13 SI�

events, it is found that a rapid reduction of the overallauroral luminosity is quite common. The reduction of aurorais found to be associated with a reduction of the diffuse typeof electron precipitation primarily in the keV energy rangeand above. The reduction is more significant in the daysidethan in the nightside part of the oval and sometimes revealsa day-to-night fading effect, with a time scale of �10 min.We proposed that adiabatic decompression and magneto-sphere reconfiguration are likely the cause of the reductionof auroral precipitation, whereas Fermi acceleration can bethe cause of the decrease in the energy of precipitatingelectrons.

[17] Acknowledgments. We acknowledge M. Torr, who built thePolar UVI instrument, and G. Parks, the current principal investigator.The Wind plasma and magnetic field data were courtesy of K. W. Ogilvie(PI of SWE) and R. P. Lepping (PI of MFI), respectively. The ASY/SYMindices are provided by the World Data Center for Geomagnetism, Kyoto,Japan. NASA grant NNG-05GB72G issued through the Polar MissionProgram supported this work.

ReferencesAraki, T. A. (1994), A physical model of geomagnetic sudden commence-ment, in Solar Wind Sources of Magnetospheric Ultra-Low-FrequencyWaves, Geophys. Monogr. Ser., vol. 81, edited by M. J. Engebretson,K. Takahashi, and M. Scholer, pp. 183–200, AGU, Washington, D. C.

Craven, J. D., L. A. Frank, C. T. Russell, E. J. Smith, and R. P. Lepping(1986), Global auroral responses to magnetospheric compressions by

L11103 LIOU ET AL.: SI� INDUCED GLOBAL AURORA QUENCH L11103

4 of 5

shocks in the solar wind—Two case studies, in Solar Wind Magneto-sphere Coupling, edited by Y. Kamide and J. A. Slavin, pp. 367–380,Terra Sci., Tokyo.

Iyemori, T., and D. R. K. Rao (1996), Decay of the dst field of geomagneticdisturbance after substorm onset and its implication to storm-substormrelation, Ann. Geophys., 14, 608–618.

Johnstone, A. D., D. M. Walton, R. Liu, and D. A. Hardy (1993),Pitch angle diffusion of low-energy electrons by whistler mode waves,J. Geophys. Res., 98, 5959–5967.

Liou, K., C.-I. Meng, A. T. Y. Lui, P. T. Newell, and S. Wing (2002),Magnetic dipolarization with substorm expansion onset, J. Geophys.Res., 107(A7), 1131, doi:10.1029/2001JA000179.

Meng, C.-I., and K. Liou (2002), Global auroral power as an index forgeospace disturbances, Geophys. Res. Lett., 29(12), 1600, doi:10.1029/2001GL013902.

Sato, N., et al. (2001), Enhancement of optical aurora triggered by the solarwind negative pressure impulse (SI�), Geophys. Res. Lett., 28, 127–130.

Spann, J. F., M. Brittnacher, R. Elsen, G. A. Germany, and G. K. Parks(1998), Initial response and complex polar cap structures of the aurora inresponse to the January 10, 1997, magnetic cloud, Geophys. Res. Lett.,25, 2577–2580.

Strickland, D. J., J. R. Jasperse, and J. A. Whallen (1983), Dependence ofauroral FUV emissions on the incidence electron spectrum and neutralatmosphere, J. Geophys. Res., 88, 8051–8062.

Takeuchi, T., T. Araki, A. Viljanen, and J. Watermann (2002), Geomagneticnegative sudden impulses: Interplanetary causes and polarization distri-bution, J. Geophys. Res., 107(A7), 1096, doi:10.1029/2001JA900152.

Torr, M. R., et al. (1995), A far ultraviolet imager for the international solar-terrestrial physics mission, Space Sci. Rev., 71, 329–383.

Vorobyev, V. G. (1974), SC-associated effects in auroras, Geomagn. Aeron.,14, 72–74.

Zhang, Y., L. J. Paxton, T. J. Immel, H. U. Frey, and S. B. Mende (2002),Sudden solar wind dynamic pressure enhancements and dayside detachedauroras: IMAGE and DMSP observations, J. Geophys. Res., 108(A4),8001, doi:10.1029/2002JA009355.

Zhou, X., and B. T. Tsurutani (1999), Rapid intensification and propagationof the dayside aurora: Large scale interplanetary pressure pulses (fastshocks), Geophys. Res. Lett., 26, 1097–1100.

Zhou, X., R. J. Strangeway, P. C. Anderson, D. G. Sibeck, B. T. Tsurutani,G. Haerendel, H. U. Frey, and J. K. Arballo (2003), Shock aurora: FASTand DMSP observations, J. Geophys. Res., 108(A4), 8019, doi:10.1029/2002JA009701.

�����������������������K. Liou, C.-I. Meng, P. T. Newell, and T. Sotirelis, Johns Hopkins

University Applied Physics Laboratory, Laurel, MD 20723, USA.(kan.liou@jhuapl.edu)

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