Relationship between cusp-region ion outﬂows and east-west magnetic ﬁeld ﬂuctuations in Southern and Northern Hemispheres

A number of interdependent conditions and processes contribute to ionospheric-origin energetic ion outﬂows. Due to these interdependences and the associated observational challenges, energetic ion outﬂows remain a poorly understood facet of atmosphere-ionosphere-magnetosphere coupling. Here we demonstrate the relationship between east-west magnetic ﬁeld ﬂuctuations ( $ \ Delta B {\ textrm { EW }} $ ) and energetic outﬂows in the magnetosphere-ionosphere transition region. We use dayside cusp-region FAST satellite observations made at apogee ( $ \ sim $ 4200-km altitude) near fall equinox and solstices in both hemispheres to derive statistical relationships between ion upﬂow and ( $ \ Delta B {\ textrm { EW }} $ ) spectral power as a function of spacecraft-frame frequency bands between 0 and 4 Hz. Identiﬁcation of ionospheric-origin energetic ion upﬂows is automated, and the spectral power $ P { EW } $ in each frequency band is obtained via integration of $ \ Delta B {\ textrm { EW }} $ power spectral density. Derived relationships are of the form $ J {\ parallel,i } = J { 0,i } P { EW } ˆ \ gamma $ for upward ion ﬂux $ J {\ parallel,i } $ at 130-km altitude. The highest correlation coeﬃcients are obtained for spacecraft-frame frequencies $ \ sim $ 0.1–0.5 Hz. Summer solstice and fall equinox observations yield power law indices $ \ gamma \ simeq $ 0.9–1.3 and correlation coeﬃcients $ r \ geq 0.92 $ , while winter solstice observations yield $ \ gamma \ simeq $ 0.4–0.8 with $ r \ gtrsim 0.8 $ . Mass spectrometer observations reveal that the oxygen/hydrogen ion composition ratio near summer solstice is much greater than the corresponding ratio near winter. These results thus reinforce the importance of ion composition in any outﬂow model. If observed $ \ Delta B {\ textrm { EW }} $ variations are purely spatial and not temporal, we show that spacecraft-frame frequencies $ \ sim $ 0.1–0.5 Hz correspond to perpendicular spatial scales of several to tens of kilometers.


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In this study we consider the relationship between cusp-region upward ion fluxes 92 and east-west magnetic field perturbations ∆B EW in nearly arbitrary frequency bands, 93 in both hemispheres during winter and summer. We also show that ion composition is 94 likely an important factor in predicting energetic outflow fluxes. In Section 2 we describe 95 FAST satellite ion and magnetic field (B-field) measurements and how we process these 96 quantities to calculate average upward ion fluxes and east-west B-field fluctuations as 97 a function of spacecraft-frame frequency band. We apply our methodology to the FAST 98 observations that S05 and B11 considered, and compare our results to theirs. In Section 3 99 we use our methodology, together with four different groups of FAST observations made 100 between December 1996 and January 1999, to obtain statistical relationships between 101 average upward ion flux and ∆B EW for nearly arbitrary spacecraft-frame frequencies be-102 tween 0 Hz and 4 Hz. In Section 4 we discuss and summarize the results in Section 3, 103 including how our methodology could be applied to current satellite missions; we dis-104 cuss the role of ion composition in these as well as previous results; and we show that    -7-manuscript submitted to JGR: Space Physics fluxes of the lower-energy, ionospheric-origin ion population are intense (dJ E /dE 10 8 eV/cm 2 -164 s-sr-eV). This ionospheric population does not appear in the "downward ion" spectro-165 gram (Figure 1c), which is the ion energy-time spectrogram that results from averaging 166 over all earthward pitch angles |θ| < 90 • . 167 We wish to exclude the contribution from magnetospheric ions to the calculated 168 ionospherically sourced upward ion flux. To achieve this, S05 and B11 manually inspected 169 the ion energy spectrogram from each cusp pass and visually determined a cutoff energy.

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They then integrated the observed ion distributions up to this cutoff energy and over all 171 pitch angles.

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Attempting to exactly reproduce the results of S05 and B11 is difficult because they 173 do not state the ion cutoff energies that were used for each orbit. We have alternatively  anti-earthward pitch angles to obtain an "upward ion" energy spectrogram (e.g.,
where J ,i is the predicted upward ion flux after mapping to 130-km altitude, γ is the

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While we believe these aspects are significant, our methodology and data sets are nev-350 ertheless subject to their own limitations.
where v F,⊥ is the perpendicular speed of FAST and V is the poleward plasma convection speed. Thus the spacecraft-frame frequency f sc corresponds to a perpendicular spatial scale The rightmost column of Table 3 shows the resulting range of perpendicular spa-492 tial scales at FAST apogee for each group of orbits. If the above-stated assumptions are 493 valid, east-west field variations with perpendicular spatial scales of order tens of kilome-494 ters are associated with ion outflow.

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As a consistency check, applying equation 2 to the range of scale sizes L ⊥ = 10-496 42 km corresponding to Group 2 in Table 3 shows that if FAST had been moving pole-  Table 3 suggests the above assumptions are at least plausible. In summary, Figures S1-S4 in this Supporting Information show that when FAST observations are made at altitudes primarily above 3800 km (Orbit Groups 1 and 2, Figures S1 and S2), r coefficients are largely unchanged when we relax the restriction to observations made at altitudes at or above 3800 km. In contrast, when a large fraction of FAST observations are made at altitudes below 3800 km (Orbit Groups 3 and 4, Figures S3 and   S4), r coefficients are largely reduced when we relax the restriction to observations made at altitudes at or above 3800 km.
Text S1. Figure S1 shows analysis of the 33 NH cusp-region passes during September 23-26, 1998 (Group 1) presented in section 3.1. Figure S1a shows From Figure S1a it is apparent that all observed ion outflow occurs above 3800-km altitude, while ∆B EW measurements are made both above and below 3800-km altitude.
Relaxing the 3800-km altitude restriction for Group 1 orbits therefore provides no additional ion observations in the calculation of average ion flux for each Group 1 orbit, and a relatively small number of additional ∆B EW measurements for the calculation of each ∆B EW power spectral density (PSD). We therefore expect little or no difference between r coefficients calculated with the 3800-km altitude restriction ( Figure S1b, reproduced from  Figure S1c, which are calculated with the 3800-km altitude relaxed As expected, there is little difference between r coefficients in Figure S1b (reproduced from Figure 3c in the main article), and r coefficients in Figure S1c, which are calculated with the 3800-km altitude relaxed.
Comparison of Figure S1b and Figure S1c indicates that the overall coefficients of r are in fact slightly higher when the additional B-field measurements from below 3800-km are X -4 HATCH ET AL.: SH/NH OUTFLOWS AND E-W FIELD FLUCTUATIONS included in the analysis of Group 1 orbits. In specific Figure S1c shows that the region for which r ≥ 0.9 is somewhat larger than the r ≥ 0.9 region in Figure S1b. The morphologies shown in each Figure are nevertheless largely the same. Figure S2 shows, in a format identical to that of Figure S1, analysis of the 38 NH cuspregion passes between December 30, 1996 and January 7, 1997 (Group 2) presented in section 3.2. Figure S2a shows that, for Group 2 orbits, ion outflow occurs above 3800-km altitude, while ∆B EW measurements are made both above and below 3800-km altitude.
Similar to the Group 1 orbits just discussed, relaxing the 3800-km altitude restriction for Group 2 orbits provides no additional ion observations in the calculation of average ion flux for each Group 2 orbit, while a relatively small number of ∆B EW measurements are added to each ∆B EW PSD calculation.
Thus, as with Group 1 orbits, there is little difference between r coefficients in Figure S2b (reproduced from Figure 4c in the main article), and r coefficients in Figure S2c, which are calculated with the 3800-km altitude relaxed. Comparison of Figures S2b and S2c indicates that r is almost unchanged when the additional B-field measurements from below 3800-km are included in the analysis of Group 2 orbits. Figure S3 shows, in a format identical to that of Figure S1, analysis of the 32 SH cuspregion passes during January 8-15, 1999 (Group 3) presented in section 3.3. Figure S3a shows, for Group 3 orbits, the occurrence of ion outflow and measurements of ∆B EW over altitudes between ∼3100 km and apogee near 4150 km.
In contrast to Group 1 and Group 2 orbits, relaxing the 3800-km altitude restriction for Group 3 orbits provides many additional ion observations and ∆B EW measurements in the calculation of average ion flux and ∆B EW PSD, respectively, for several Group 3 orbits.
Comparison of the distribution of r coefficients in Figure S3b (reproduced from Figure 5c in the main article) with r coefficients in Figure S3c, which are calculated with the 3800km altitude relaxed, reveals large differences. First, r coefficients in Figure S3c are nearly everywhere lower than corresponding r coefficients in Figure S3b. Second, r coefficients in Figure S3c decrease more rapidly with increasing frequency lower bound f bot (or "start frequency," shown on the y axis) for f bot 0.2 Hz. Figure S4 shows, in a format identical to that of Figure S1, analysis of the 29 SH cuspregion passes between May 24 and June 5 in 1998 (Group 4) presented in section 3.4. Figure S4a shows, for Group 4 orbits, the occurrence of ion outflow and measurements of ∆B EW over altitudes between ∼3200 km and apogee near 4150 km.
As with Group 3 orbits, relaxing the 3800-km altitude restriction for Group 4 orbits provides many additional ion observations and ∆B EW measurements in the calculation of average ion flux and ∆B EW PSD, respectively, for several Group 4 orbits.
Comparison of the distribution of r coefficients in Figure S4b (reproduced from Figure 6c in the main article) with r coefficients in Figure S4c, which are calculated with the 3800km altitude relaxed, reveals very large differences. In particular the range of f bot values for which r is highest (r 0.85) in Figure S4b correspond to r 0.5 in Figure S4c. The r coefficients in Figure S4c are nearly everywhere lower than corresponding r coefficients in Figure S4b. The r coefficients in Figure S4c for f bot 2 Hz, while universally less than 0.7, are in many places higher than corresponding values in Figure S4b.  Figure S4. Comparison of the frequency-band analysis for 29 SH cusp-region passes between May 24 and June 5 in 1998, in the same format as Figure S1. Panel b corresponds to Figure 6c in the main article. All measurements are restricted to dayside MLTs and to -87 • through -60 • MLat.