Research ArticleSPACE SCIENCES

Assessing the global Alfvén wave power flow into and out of the auroral acceleration region during geomagnetic storms

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Science Advances  26 Jun 2019:
Vol. 5, no. 6, eaav8411
DOI: 10.1126/sciadv.aav8411

Figures

  • Fig. 1 Solar cycle and experimental setup.

    (A) Top: Sunspot number versus time. The period under investigation in this study is shaded green, and the thick line is the averaged curve. Bottom: Global view of the initial development of the aurora during the Bastille Day storm observed by the IMAGE spacecraft’s Wideband Imaging Camera [from (4)]. The time in the lower right-hand corner of each image is in universal time. The arrow above the three images points at the date in the solar cycle when this storm occurred. (B) Auroral current circuit with key regions and spacecraft location [modified from (60)]. (C) Hypothetical orbital ellipsoid of the Polar satellite crossing different plasma regions of Earth’s magnetosphere during 1 year. The colored regions (yellow and red) illustrate regions through which Alfvén waves travel toward or away from the ionosphere in one particular instance. The hole in the ellipsoid reveals Earth with the aurora borealis. Green lines with arrows represent magnetic field lines.

  • Fig. 2 Morphology of in situ Poynting flux during nonstorm and storm periods.

    (A) Global distribution of average wave Poynting flux, as a function of magnetic local time (MLT) and latitude, flowing toward Earth (labeled “Downward”). The view is onto the polar-auroral region, with the magnetic north pole in the center. The upper and lower halves correspond to dayside and nightside, respectively. The electric and magnetic field data for calculating Poynting fluxes were measured between 4 and 7 RE (geocentric distance) in the Northern Hemisphere over a period of 6 years (see Fig. 1B for the orbital ellipsoid). The Poynting fluxes shown were scaled along converging magnetic field lines to ionospheric altitudes (100 km) under nonstorm conditions (−20 nT < Dst < 0 nT). Each bin covers 2° magnetic latitude times 0.75 hour local time. Circles represent magnetic latitudes (60°, 70°, and 80°); radial lines indicate local times (e.g., 6, 12, 18, and 24). The black circle in the center was not analyzed. The number at the lower left corner of each panel is the total hemispherical (>60°) power for each distribution. (B) Same as (A) but under storm conditions (Dst < −40 nT). For a few bins (black), Polar did not encounter storm conditions; hence, no Poynting flux values are shown. The color scale is the same as in (A) for comparison purposes, which, however, causes some bins to saturate at the highest value. (C and D) Corresponding average Poynting flux distributions flowing toward the magnetosphere (labeled “Upward”).

  • Fig. 3 Tracing Poynting flux along Polar’s orbit during a period of 28 days, which includes a moderate storm and a major storm.

    (A) Peak values of Poynting flux binned by 0.5° magnetic latitude (y axis) versus days (x axis) and mapped using the T01 model. Each column represents the section of a Polar orbit within 50° and 90° magnetic latitude. Only values obtained when Polar was on the dayside are included. The Dst curve [from (B)] is overlaid (white line) with arbitrary scale for illustrative purposes. (B) Dst index versus time, with labels indicating a moderate and a major storm. Numbered vertical lines (1 to 5) refer to the features in the panels above and below (see text). The inset shows Polar’s daily orbits during the time period shown here projected onto the latitude-local time plane. (C) Same data quantity as in (A) but for the nightside and with the Dst curve overlaid. (D) Same data quantity as in (A) but for the nightside. However, the mapping was performed using the dipole model for comparison.

  • Fig. 4 Alfvén wave power along MLT.

    (A) Storm condition (Dst < −40 nT). (B) Nonstorm condition (−20 nT < Dst < 0 nT). In both panels, the solid line shows mean values, and the green-shaded area shows 1 SD away from the mean.

  • Fig. 5 Morphology of in situ peak Poynting flux during storm periods.

    The global distribution of peak wave Poynting flux during storms (Dst < −40 nT) flowing toward Earth as measured at high altitude (4 to 7 RE geocentric) in the Northern Hemisphere obtained from 6 years of Polar measurements and scaled along converging magnetic field lines to ionospheric altitudes (100 km). Each bin covers 2° magnetic latitude times 0.75 hour local time. Circles represent magnetic latitudes (60°, 70°, and 80°); radial lines indicate local times (e.g., 6, 12, 18, and 24). The black circle in the center was not analyzed. For a few bins (black), Polar did not encounter storm conditions; hence, no Poynting flux values are shown. The two dashed oval-shaped curves delineate the statistical auroral oval for geomagnetically active periods (15).

Tables

  • Table 1 Comparison of global powers for the Northern Polar Region.

    SatelliteNonstormMain phaseRecovery
    phase
    Polar (above AAR)*
      Alfvén wavedown (GW)2.5910.056.29
      Alfvén waveup (GW)0.674.072.62
      Net deposition (GW)1.925.983.67
    FAST (below AAR)
      Alfvén wave (GW)0.542.621.75
      Electron precipitation
    (GW)
    0.915.084.27
      Net deposition (GW)§1.457.706.02

    *Polar study period: January 1997 to December 2002.

    †Net deposition at Polar is not necessarily the true deposition below the AAR (see text for explanation).

    ‡FAST study period: October 1996 to November 1999 [from (25)].

    §Calculated under the assumption that all Alfvén wave power at FAST flows toward the ionosphere (i.e., no upflowing Alfvénic power).

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