SAID: A turbulent plasmaspheric boundary layer

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GEOPHYSICAL RESEARCH LETTERS, VOL. 37, L07106, doi:10.1029/2010GL042929, 2010

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SAID: A turbulent plasmaspheric boundary layer E. V. Mishin,1 P. A. Puhl‐Quinn,2 and O. Santolik3,4 Received 17 February 2010; revised 3 March 2010; accepted 11 March 2010; published 8 April 2010.

[1] This paper presents novel features of subauroral ion drifts (SAID) observed from a unique conjunction of the Cluster, DMSP, and Polar satellites, including the discovery of SAID‐related plasma waves. These observations confirm and expand on our proposed concept of the SAID channel being a turbulent boundary layer, formed via a short circuit of the substorm‐injected plasmoid by the plasmasphere. We show that SAID formation is related to enhanced lower hybrid/fast magnetosonic waves. Their excitation leads to anomalous circuit resistivity and magnetic diffusion, similar to the well‐documented plasmoid‐magnetic barrier problem, including impulsive penetration at the magnetopause. Citation: Mishin, E. V., P. A. Puhl‐Quinn, and O. Santolik (2010), SAID: A turbulent plasmaspheric boundary layer, Geophys. Res. Lett., 37, L07106, doi:10.1029/2010GL042929.

[4] MPQ showed that the existing models [e.g., Anderson et al., 2001; De Keyser, 1999] are unable to explain the overall features and proposed a short circuit of substorm‐ injected hot plasmoids over the plasmasphere. MPQ also assumed that the (anomalous) circuit resistivity and magnetic diffusion at the front arise from excited plasma turbulence, as with the plasmoid‐magnetic barrier problem [e.g., Mishin et al., 1986; Hurtig et al., 2005]. However, the crucial Cluster wave observations have not been explored; nor have 1‐keV electron counts, the only Cluster hot electron data acquired near perigee. In this paper, we present the first observations of SAID‐related plasma waves and expand on the SAID mechanism exploring a unique Cluster‐DMSP‐ Polar conjunction.

2. Observations 1. Introduction [2] SAID are latitudinally‐narrow (Ds ≤ 1° Lat), streams of westward subauroral convection driven by locally‐ enhanced poleward electric fields Es. Before Cluster, such events were documented mainly at altitudes h < 1.5RE [e.g., Anderson et al., 2001], with only a few in the near‐equatorial magnetosphere [Maynard et al., 1980; Burke et al., 2000]. Puhl‐Quinn et al. [2007, hereafter PQ] and Mishin and Puhl‐ Quinn [2007, hereafter MPQ] presented first magnetically‐ conjugate Cluster‐DMSP observations of substorm SAID near the magnetic equator (h ’ 3.3RE and 3.5RE) and in the ionosphere on 8 April 2004 and 18 March 2002, respectively. Making the latter event truly unique, Polar crossed the SAID channel at h ≈ 0.9RE. [3] PQ and MPQ found that (A) the SAID outer (tailward) boundary coincides with the cold plasma density jump at the plasmapause and hot (" > 1 keV) electron precipitation cutoff, (B) the inner (earthward) boundary marks the drop in the hot ion flux, (C) in the ionosphere, Es peaks near either the density peak or the trough’s poleward edge, (D) the SAID‐related downward field‐aligned current (FAC) concentrates near the outer side, (E) the mapped magnetospheric FAC exceeds its in situ ionospheric counterpart, and (F) the SAID channels are at rest between subsequent crossings by the Cluster satellites.

1 Space Vehicles Directorate, Air Force Research Laboratory, Hanscom AFB, Massachusetts, USA. 2 Atmospheric and Environmental Research, Inc., Lexington, Massachusetts, USA. 3 Department of Space Physics, Institute of Atmospheric Physics, Prague, Czech Republic. 4 Also at Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic.

Copyright 2010 by the American Geophysical Union. 0094‐8276/10/2010GL042929

[5] PQ and MPQ described in detail the 8‐Apr‐04/ 18‐Mar‐02 substorm SAID events. In the magnetosphere, the Cluster (C1–C4) spacecraft observed southern hemisphere SAID channels of the width Ds ∼ 0.1RE and max (Es) ≈ 25/10 mV/m near 61.3°/63.2° ILat (Ls ’ 4.3/4.9) and 21.9/ 23.1 MLT. Their northern (outbound) counterparts, encountered ∼40/70 min later, had almost identical features. Herein, we focus on the 18‐Mar‐02 C1/C4 southern (inbound) feature near 10:15 UT, as representative of the overall Cluster observations. About 6 min earlier, during an inbound pass Polar crossed the northern SAID channel at Ls ≈ 5.3 and 22.7 MLT, magnetically‐conjugate to the Cluster northern‐hemisphere channels. [6] Figure 1 presents the 18‐Mar‐02 southern SAID event from C1 and C4 (cf. MPQ Figure 1). The time lag of the C4 satellite vs. C1 (≈0.4 min) has been taken into account. Shown are (a) the C1/C4 outward EFW electric field Ex in the inertial frame, (b) C4/0.03–38 keV omnidirectional H +‐fluxes from the CIS‐CODIF instrument (the potential energy eDF = −e∫Exdx [eV] is superimposed), (c) C1/EDI 1‐keV electron counts (thick line), DMSP F14 scaled flux of 1‐keV electrons (dashed), and the cold plasma density n0 (thin), (d) and (e) C4/STAFF frequency‐time spectrograms for electric and magnetic fields in (mV/m)2/Hz and nT2/Hz, respectively (in log scale). The flux scaling means the magnetic lensing and reduction by a matching factor (here ≈102). We use the coordinates (x, y, z) = (across L‐shell/ outward, eastward, B0‐aligned), where B0 is the IGRF (International Geomagnetic Reference Field) 2000 model field [Olsen et al., 2000]. [7] In Figure 1c, n0 was derived from the probe‐to‐ spacecraft potential p by Santolik et al. [2004]. It matches the overlapping values (6–80 cm−3) from the WHISPER instrument within 10% (cf. PQ, Figure 2). It is seen that both electron curves decline when n0 jumps over ≈10 cm−3, though the EDI counts’ drop is slightly delayed. We note that such correlation between p (n0) and EDI counts is

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Figure 1. (a) The outward electric component Ex from the EFW instrument, (b) C4/0.03–38 keV omnidirectional H + fluxes in cm−2s−1 sr−1 keV−1 (the dashed line shows the potential energy eDF in eV), (c) C1/EDI 1‐keV electron counts (thick line), scaled DMSP F14 flux of precipitating 1‐keV electrons (dashed), and the cold plasma density in cm−3 (thin), (d) the C4/STAFF frequency‐time spectrograms for electric, in (mV/m)2/Hz, and (e) magnetic, in nT2/Hz, fields. Horizontal lines indicate flhr (solid) and 10th, 4th, and 2nd ion gyroharmonics (dashed). Color codes in log scale for the wave spectrum and H + fluxes are given to the right of the spectrograms. Hereafter, the vertical dashed (dotted) line indicates the SAID outer boundary (center). typical of all (eighteen) SAID events found in the Cluster database 2002–2007. [8] Figure 1d shows that the wave activity is abruptly enhanced inside the channel.q Horizontal lines indicate the ffiffiffiffiffiffiffiffiffiffiffiffiffi lower hybrid resonance flhr = 1þffce2fci=f 2 ≈ 230 Hz (solid) and ce pe the 10th, 4th, and 2nd harmonics of the H +‐ion gyrofrequency fci = wci/2p ≈ 5.4 Hz (dashed), derived from the observed magnetic pffiffiffiffiffi field (B ≈ 370 → 350 nT). Here wci = eB/mic, fpe ≈ 9 n0 kHz ( fce) is the electron plasma (cyclotron) frequency, e/mi is the ion charge/mass, and c is the speed of light. The most h intense i broadband waves of f ≤ 2flhr and pffiffiffiffi ≤ 0.1 (≤1 in the 8‐Apr‐04 event) arise amplitudes dEf mmV Hz in the entry layer and extend to the center. Interestingly, the waves in the entry layer are mainly electrostatic, while the magnetic component (panel e) rises near and earthward of the center. Additionally, emissions near 2 kHz (near 3.5 kHz on 8‐Apr‐04) emerge and at ≤2fci intensify across the channel (∼10:1520 → :1810). Note that the same spatio‐ temporal and spectral features of the wave activity are observed also by the wideband Cluster/WBD receiver (not shown). [9] Figure 2 shows the northern SAID channel from Polar: (a) the outward electric field, (b) the energy‐time spectro-

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gram for 0.012–18 keV electrons (the scaled DMSP 1‐keV electron flux is overlaid) and (c) ion omnidirectional differential fluxes in cm−2s−1sr−1keV−1 from the HYDRA instrument, (d) the eastward component of the magnetic (thick), field By (thin line) and its 12‐s sliding average  hB  yi 0:75 @  (e) the FAC density (positive downward) jk A Vx @t By m2 with the outward satellite speed Vx in km/s, and (f) the plasma density. [10] The ion distribution in Figure 2b resembles that in Figure 1b and the Es‐peak matches that mapped from the Cluster northern event. The downward FAC concentrates near the outer side (cf. (D)). Consistent with (E), its magnitude is ’0.56 of that mapped from Cluster (cf. Figure 3e and MPQ Figure 3d). This implies that the SAID‐related FAC is partially closed above 0.9RE, i.e. its (kB) scale‐ length is lcp ]3RE. Lastly, the in situ and DMSP 1‐keV fluxes in panel b coincide. In fact, the entire hot electron population closely resembles that mapped from DMSP (cf. MPQ Figure 1e), so it is not surprising that (A) is also satisfied. [11] Thus, the Polar observations confirm the SAID features from the Cluster‐DMSP conjunction. In addition, from the Polar/HYDRA data it follows that outside the channel (at 10:18:10 UT) and extrapolated outward (upstream). [13] Lastly, Figure 3e shows the FAC density jk and the @ nA azimuthal (positive eastward) currents, jy ≈ 750 Vx @t dBg m2 , calculated with dBPi+e (diamagnetic) and 6‐s sliding average hdBi (cf. MPQ Figure 3d). As ∣dBz∣ ’ ∣dBy∣ ≥ 10 ∣dBx∣ inside the channel and the channel radial speed is ≤Vx/6 [MPQ], the values of jk,y are accurate to 20% or less.

3. Discussion and Conclusions [14] The in situ wave data do confirm that enhanced plasma waves accompany the SAID events. We found that the hot ion anisotropy, Q(x) = Ti?(x)/Tik(x) − 1, increases across the channel from ≈0.1 to 0.8 and that their (?B) distribution becomes ring‐like at "min ≤ " ≤ "max. Here "min ("max) increases (decreases) across the channel (see Figure 1b). The ring distribution easily excites ion cyclotron and fast magnetosonic (oblique whistler) waves [e.g., Horne et al., 2000]. Particularly, higher gyroharmonics f ≥ 10fci pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi are excited if vmax = 2" =mi exceeds the local Alfvén max pffiffiffiffiffi speed vA ≈ 104/ n0 km/s (recall that hBi ≈ 360 nT) or n0 exceeds nA ≈ 50 cm−3. This conforms to the observed

spectral features near and earthward of the SAID center (Figures 1d–1e). However, the ring instability hardly explains the wave activity in the entry layer of the thickness dz ≈ 90 km (Figure 3e), where n0 < nA. Nor is the lower‐ hybrid drift instability driven by the ion diamagnetic current near the inner edge [cf. LaBelle and Treumann, 1988; Mishin and Burke, 2005]. [15] MPQ described the SAID formation in terms of a short circuiting over the plasmasphere of a (hot) plasmoid of the width w (along y) moving across B (≈z) with the bulk speed V0kz(≈−x) due to the self‐polarization field Ep = −yV0B/c at the front. The polarization charge builds up at the edges in the charge layer dy ∼ r0v2Au/c2 within the time t p ∼ dy/V0, where vAu = vA(Nu) and r0 = V0/wci [Schmidt, 1960]. Forpthat, the plasmoid density Nh must exceed ffiffiffiffiffiffiffiffiffiffiffiffiffiffi Nmin ≈ B2/ me mi c4 . Indeed, the upstream value Nh = Nu ≈ 1.5 cm−3 (Figure 3c) is ≈5Nmin. Notice, the plasmoid propagation from the substorm onset in ≤20 min requires V0 ≥ 10 km/s (cf. MPQ). [16] In collisionless stable plasmas, the condition for a plasmoid to penetrate a magnetic barrier is w < 12r0Bd/(Bd − Bu) [Gunell et al., 2008]. Here Bd (Bu) is the barrier or downstream (upstream) magnetic field. However, a number of experimental and theoretical studies show that the downstream field promptly permeates the plasmoid due to an instability in the front, thereby relaxing the restraint to w [e.g., Mishin et al., 1986; Wessel et al., 1990; Hurtig et al., 2005; Gunell et al., 2008]. The value of w can be estimated from the current closure condition divj = 0 earthward of the SAID center, where jx ’ 0. This gives w ∼ lk · jy/jk ∼ lk, as here jy ∼ jk (Figure 3e). Taking lk ∼ lcp yields w < 3RE, which agrees with MPQ’s inference of w ’ 1.5·104 km (’2 MLT hours). [17] Figure 3d shows that upstream of the channel DBu ≈ DBPi+e, i.e. the downstream field does permeate the plasmoid. Here, the mean value of Ey is Eu ≈ −0.45 mV/m or Vu ≈ 1.4 km/s, while n0 ≥ 4 cm−3. That the hot electrons are not halted means that the plasmoid slows down due to partial depolarization in the background plasma [cf. Wessel et al., 1990, Figure 3]. Polarization shorting occurs when t p exceeds the time t k of the (kB) motion of plasma electrons into the charge layer dy. Wessel et al. [1990] estimated the critical background density in a collisionless plasma as ncs ’ p3mec2N2u/B2u ≈ 4 cm−3. However, in collisional/turbulent !2 plasmas the (kB) electron mobility and jk = 4vpea dEk are limited. Here n a is the effective collision frequency of the cold electrons and dE = −rF is the self‐consistent electric field. As a result, t k and nc increase [cf. Zakharov et al., ’ 10 cm−3 2002]. Indeed, the observed value is n(obs) c (Figure 1c) (cf. MPQ). Although the rigorous solution for this problem has not yet been found, q Rozhanskii [1986] ffiffiffiffiffiffiffiffiffiffi obtained the critical density nca ’ Nu m4me i Va wu . This gives at n a ’ 2 · 10−4wpe [cf. Mishin et al., 1988]. nca ’ n(obs) c [18] In the cited ‘barrier’ studies, the magnetic diffusion rate in the unstable plasmoid front, D ∼ n el2s , is defined pffiffiffi by the effective collision frequency n e and ls ≈ 5/ n km, where n = max (n0, Nh). A typical value of n e is about wlhr. The instability is driven by the hot electron (diamagnetic) current jye. Sotnikov et al. [1980] developed a nonlinear theory of such instability, arriving at

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e ’

!2pe !ce

rffiffiffiffiffiffiffiffiffiffi 2 e 10mi E ; me 8n0 Ti

ð1Þ

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e is the r.m.s. where Ti is the cold ion temperature and E e≈ amplitude. In the entry layer, where jye ≈ 17 nA/m2 and E 1 mV/m (Figures 3b and 3e), equation (1) yields n e ≈ 3wlhr. [19] Finally, Figures 3d–3e show that DBPi+e and DB (and the corresponding currents) differ in the entry layer. This discrepancy results from the assumption that inside the channel Pe mimics the EDI counts, albeit ≤1‐keV electrons contribute slightly to Pe. In fact, due to the continuing influx, the hot electrons should accumulate near the boundary, thereby increasing the pressure. In a steady state, the density/pressure buildup is balanced by precipitation and azimuthal rdB‐drift. As there is no increase in the Polar/ DMSP precipitating hot electron flux near the boundary, the buildup process should be limited mainly by the diamagnetic drift. The observed value of D(dBP) ≈ −3.5DP[nPa] ≈ −1 nT gives DPe ≈ 0.3 nPa, i.e. DPe/Peu ≈ DNe/Nu ≈ 1/3. From the continuity equation we get the eastward drift nAspeed  ’ 50V and the westward current ∣j vy ’ Vuw/3d z u ye  m2 ∣ ’ ’ 20, in good agreement with jye in the entry 10NuVu km s layer (Figure 3e). [20] In conclusion, we have presented new observations of substorm SAID from a Cluster‐DMSP‐Polar conjunction, revealing the SAID channel as a turbulent boundary layer. It is formed when a substorm‐injected plasmoid is short‐ circuited by the plasmasphere. We found that the observed lower hybrid/fast magnetosonic turbulence creates anomalous magnetic diffusivity, as in the plasmoid‐magnetic barrier problem. These results can be helpful for understanding the mechanism of impulsive penetration at the magnetopause [e.g., Lemaire and Roth, 1991]. [21] Acknowledgments. EVM was supported by the Air Force Office of Scientific Research. We thank W.J. Burke for comments and H. Laakso for making us aware of the Polar SAID event. The Cluster FGM Prime Parameter, EFW, EDI, CIS‐CODIF, WBD, and STAFF datasets were obtained through the Cluster Science Data System and the ESA Cluster Active Archive. The Polar MFE, EFI, and HYDRA datasets were obtained through the NASA CDAWeb data service. OS was supported by the GACR grant 205/10/2279.

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