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Observations of substorm auroral features in the conjugate hemispheres

- 2007 aper - Nature paper - OC boundaries Due to the precession of the Polar spacecraft long time series of imaging data from the southern hemisphere are obtained from imagers on board Polar. At the same time IMAGE FUV provide images from the northern hemisphere. We have identified 6 substorms where we have observations from both hemispheres. The figure to the right shows the substorm onset on September 13, 2001, where the onset location in the northern hemisphere (IMAGE/FUV WIC) is at 21 MLT, while the onset in the southren hemisphere (Polar VIS Earth camera) is located at 22:30 MLT. The duskward displacement (1.5 hours) of the southern substorm onset is attributed to the IMF influence on the magnetotails configuration. The tension force on interconnected (open) field lines associated with a positive IMF By component will twist the magnetotail, explaining the observed displacement of substorm onset locations.

[pdf] N. Østgaard, S. B. Mende , H. U. Frey , T. J. Immel, L. A. Frank, J. B. Sigwarth, T. J. Stubbs. Interplanetary magnetic field control of the location of substorm onset and auroral features in the conjugate hemispheres. J. Geophys. Res., Vol. 109, No. A7, A07204, 10.1029/2003JA010370, 2004.

After extending the dataset to 15 substorm auroral features we were able to identify a possible dipole tilt affect which act as a secondary controlling factor (after the IMF clock angle) of the asymmetric location of auroral features during substorms. Our result can be explained by a stronger FAC in the winter hemisphere, leading to a larger eastward displacement of the magnetic footpoint in the dark hemisphere. A comparison with durrent magnetic field models (T96 and T02) shows that the implementation of a penetrating field (i.e., the IMF) has observational support. The model, however, underestimate thios effect by an order of magnitude.

Observations of cusp precipitation in the conjugate hemispheres

When the IMF connect with the Earths magnetic field in a process called reconnection, the solar wind particles following the field lines can deposite their energies directly in the Earths upper atmosphere on the dayside. Both electrons and protons will produce an auroral spot at the footpoint of this newly opened or reconnected field line, which now has one footpoint on the Sun and the other on the Earth. The movie is an animation of this process showing the auroral spot (called the cusp spot) created by the solar wind particles. The aurora you see is real data from IMAGE.

As shown in this movie, a strongly northward IMF leads to a reconnection site at the Earths high latitude lobe. The cusp aurora it produces is clearly distuingishable from the auroral oval. The cusp aurora is expected to occur simultaneously in both hemisphere, but not necessarely at the same locations.


We have found a short timeinterval when this cusp spot was imaged simultaneously in the conjugate hemispheres by IMAGE FUV (northern hemisphere) and Polar VIS Earth camera (southern hemisphere). These observations give indisputable evidence of how the IMF and dipole tilt control the energy and momentum transfer from the solar wind into the Earth's magnetosphere. The cusp spots are strongly asymmetric both in longitude and latitude. The longitudinal shift of the cusp aurora is controlled by the IMF By component, while the latitudinal shift is consistent with a dipole tilt effect. These new discoveries, enabled by simultaneous imaging by two different satellites, also demonstrate the potential to examine differences in reconnection rate in the conjugate hemispheres. We also observed an occurence of a non-conjugate theta aurora, consistent with earlier findings: Theta aurora project

N. Østgaard, S. B.Mende, H. U. Frey & J. B. Sigwarth Asymmetries of the reconnection spot in the conjugate hemispheres, Geophys. Res. Lett., in press, 2005.
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[pdf] N. Østgaard, N. A. Tsyganenko, S. B. Mende , H. U. Frey , T. J. Immel, M. Fillingim, L. A. Frank, J. B. Sigwarth. Observations and model predictions of substorm auroral asymmetries in the conjugate hemispheres. Geophys. Res. Lett., 32, 5, L05111, doi:10.1029/2004GL022166, 2005.

Examning the oval location on October 23, 2002, as imaged by IMAGE FUV and Polar VIS Earth camera, the oval locations in the two hemispheres have revealed an unexpected assymetry.

[pdf] T. J. Stubbs , R. R. Vondrak and N. Østgaard, J. B. Sigwarth and L. A. Frank. Simultaneous observations of the auroral oval in both hemispheres under varying conditions. Geophys. Res. Lett., Vol 32, L03103, doi:10.1029/2004GL021199, 2005.

This study was released by NASA and received a lot of attention in the scientific community world wide.

Non-conjugate observations of the theta aurora

We have also identified two events where a theta aurora is observed in one hemisphere, but not in the other. On November 5 (figure to the right), the theta was observed strong and clear in the northern hemisphere (IMAGE/FUV - 135.6 nm). Although the VIS Earth camera images (130.4 nm) were contaminated by energetic protons, it is clear that the images from the southern hemisphere do not show any theta aurora. These observations were confirmed by DMSP passes in the two hemispheres. Theta aurora is associated with northward IMF and 4 cell plasma convection patterns. We attribute the non-conjugate occurance of theta aurora to the IMF Bx control of the different rates of lobe reconnection in the two hemispheres, which is the driver of the plasma convection and shear flows, producing the electric fieds that causes the theta aurora.

This study is featured as one of the IMAGE discoveries

[pdf] N. Østgaard, S. B. Mende, H. U. Frey, L. A. Frank , J. B. Sigwarth., Observations of non-conjugate theta aurora. Geophys. Res. Lett., Vol. 30, No. 21, 2125, doi: 10.1029/2003GL017914, 2003.

Reconnection rate in the magnetotail

-- Reconnection and ionospheric signatures Dayside merging between the interplanetary and the terrestrial magnetic field couples the solar wind electric field to the Earth's magnetosphere, increases the magnetospheric convection and results in efficient transport of solar wind energy into the magnetosphere. Subsequent reconnection of the lobe magnetic field in the magnetotail transports energy into the closed magnetic field region. Combining global imaging and ground based radar measurements we estimate the reconnection rate in the magnetotail during two days of the EISCAT winter campaign of 2000/2001. Global images from the IMAGE FUV system guide us to identify ionospheric signatures of the open-closed field line boundary observed by the two EISCAT radars at Tromso (VHF) and Svalbard (ESR). Continuous radar and optical monitoring of the open-closed field line boundary is used to determine the location, orientation and velocity of the open-closed boundary and the ion flow velocity perpendicular to this boundary. The magnetotail reconnection electric field is found to be a bursty process that oscillates between between 0 mV/m and 1 mV/m with ~10-15 min periods. These ULF oscillations are mainly caused by the motion of the open-closed boundary. In situ measurements earthward of the reconnection site in the magnetotail by Geotail show similar oscillations in the duskward electric field. We also find that bursts of increased magnetotail reconnection not necessarily have any associated auroral signatures. Finally we find that the reconnection rate correlates poorly with the solar wind electric field. This indicate that the magnetotail reconnection is not directly driven, but is an internal magnetospheric process. Estimates of a coupling efficiency between the solar wind electric field and magnetotail reconnection only seems to be relevant as average on long time scales. The oscillation mode at 1 mHz corresponds to the internal cavity mode with additional lower frequencies, 0.5 and 0.8 mHz, that might be modulated by solar wind pressure variations.

[pdf] N. Østgaard, J. Moen, S. B. Mende, H. U. Frey, T. J. Immel, P. Gallop, K. Oksavik, M. Fujimoto. Estimates of magnetotail reconnection rate based on IMAGE FUV and EISCAT measurements Ann. Geophys. (Eleventh International EISCAT Workshop), 23 (1), 123-134, 2005.

Geocoronal imaging and hydrogen density profiles

The geocorona is produced when solar Lyman alpha (121.6 nm) radiation is resonance scattered by exospheric neutral hydrogen. We have shown that measurements of Lyman alpha from the IMAGE-FUV/GEO instrument can be used to give us information about the hydrogen density surounding the Earth. By assuming that the medium can be considered to be optical thin above 3.5 Re (geocentric distance) and taking into account the attenuation of solar Lyman alpha to the nightside, contribution from the inner geocorona, the phase dependent scattering cross section etc. we give empirical derived expressions for the hydrogen densities at high altitudes as a function of solar zenith angles. Such density profiles are needed to analyze the energetic neutral atom imaging data at ring current altitudes and above (HENA and MENA on IMAGE).

[pdf] N. Østgaard, S. B. Mende, H. U. Frey, G. R. Gladstone, H. Lauche. Neutral hydrogen density profiles derived from geocoronal imaging. J. Geophys. Res., Vol. 108, No. A7, 1300, doi:10.1029/2002JA009749, 2003.

Energy flow and transfer in the solar wind - magnetosphere - ionosphere system

Plasma erupted from the Sun, carrying the interplanetary magnetic field, can penetrate the magnetic shielding of the Earth through a reconnection process. This merging of field lines and penetration of particles and Poynting flux represent an energy transfer into the magnetosphere. The energy can then either be deposited "directly" into the ionosphere with a typical delay of 20 min or be stored in the magnetosphere and with a typical delay time of 60 min be dissipated during substorms. The three most important energy sinks that affect the Earths environment are energy deposition from particle precipitation, Joule heating and increase of the ring current. Additional energy sinks are the ejection of plasmoids down the tail, heating of the plasma sheet and waves and relastivistic electrons. In two papers we show how much of the available solar wind kinetic energy that is deposited in the magnetosphere-ionosphere system.

[pdf] N. Østgaard and E. Tanksanen, Energetics of isolated and stormtime substorms. Disturbances in Geospace: The Storm-Substorm Relationship, Geophysical Monograph 142, doi: 10.1029/142GM14, 2003.
[pdf] N. Østgaard, G. A. Germany, J Stadsnes, R. R. Vondrak, Energy analysis of substorms based on remote sensing techniques, solar wind measurements and geomagnetic indices. J. Geophys. Res., Vol. 107, NO. A9, 1233, doi: 10.1029/2001JA002002, 2002.

Energy deposition by precipitating particles and how it affects the ionospheric electrodynamics during substorms

Techniques: Multi-spectral global imaging is a strong tool to derive information about the energy characteristics of precipitating electrons and protons. We have shown that the combined measurements of ultraviolet emissions and X rays from space can be used to derive electron spectra in the energy range from 100 eV to 100 keV on a global scale. We have also shown that the techniques give results that are consistent with insitu particle measurements from low-altitude spacecrafts.

Energy deposition from electron precipitation as a function of geomagnetic indices: We have also examined how the hemispherical energy deposition (0.1-100 keV) relates to the geomagnetic indices AL and AE and presented simple expressions that can be used when global imaging is not availbale.

Electron precipitation and ionospheric conductances: In collaboration with the group at the University of Bergen, Norway, the same dataset have been used to show the importance of the energetic tail of the electron distribution for ionospheric parameters as Hall conductance and large scale electric fields.

Plans: Ultimately, our goal is to develop a model that gives the typical electron energy spectra as function of time, magnetic local time and magnetic latitude during substorms. Ionospheric parameters as Hall and Pedersen conductance and the ratio of the two will be part of such a study as well. This should be a highly valuable tool for modelling the MI coupling. Another goal is to estimate the relative energy contribution from electrons and protons during substorms.

[pdf] N. Østgaard, J. Stadsnes, J. Bjordal, G. A. Germany, R. R. Vondrak, G. K. Parks, S. A Cummer, D. L. Chenette, J. G. Pronko, Auroral electron distributions derived from combined UV and X-ray emissions. J. Geophys. Res., Vol.106, 26,081 - 26,090, 2001.
[pdf] N. Østgaard, R. R. Vondrak, J. W. Gjerloev, G. A. Germany, A relation between the energy deposition by electron precipitation and geomagnetic indices during substorms. J. Geophys. Res., Vol. 107, NO. A9, 1233, doi: 10.1029/2001JA002003, 2002.
[pdf] A. Aksnes, J Stadsnes, J. Bjordal, N. Østgaard, R. R. Vondrak, D. L. Detrick, T. J. Rosenberg, G. A. Germany and D. Chenette. Instantaneous ionosphericglobal conductance maps during an isolated substorm. Ann. Geophysicae, 20, 1181, 2002.
[pdf] A. Aksnes, J Stadsnes, G. Lu, N. Østgaard, R. R. Vondrak, D. L. Detrick, T. J. Rosenberg, G. A. Germany and M. Schulz. Effects of energetic electrons on the electrondynamics in the ionosphere Ann. Geophys., 2, 475 - 496, 2004.

Dynamics of local features of energetic electron precipitation examined by global X-ray imaging

  • Morning maximum delayed to substorm onset An X-ray substorm looks quite different from an UV/visible substorm (see substorm movie from July 31, 1997). A striking difference is that the X-ray substorm reveals a very clear dawnward auroral motion and shows a morning maximum of energetic electron precipitation delayed relative to substorm onset. We have studied the electron energies that are involved in this motion of auroral forms and suggested causative mechanism for this precipitation. Our conclusion is that the prolonged precipitation in the morning sector can be explained by gradient drifting electrons that interact with VLF waves. As long as the flux level of quasi-trapped electrons is high enough electrons will be scattered into the loss cone through wave-particle interaction to produce the observed prolnged maximum at dawn.

    [pdf] N. Østgaard, J. Stadsnes, J. Bjordal, R. R. Vondrak, S. A Cummer, D. L. Chenette, M. Schulz and J. G. Pronko, Cause of the localized maximum of X-ray emission in the morning sector: A comparison with electron measurements. J. Geophys. Res., Vol. 105, No A9, p. 20,869 - 20,885, 2000.
    [pdf] N. Østgaard, J. Stadsnes, J. Bjordal, R. R. Vondrak, S. A Cummer, D. L. Chenette, M. Schulz, G. K. Parks, M. J. Brittnacher, D. L. McKenzie and J. G. Pronko, Global X-ray emission during an isolated substorm - A case study. J. Atmos. Solar-Terr. Phys., Vol. 62 (10), p. 889 - 900, 2000.
    [pdf] N. Østgaard, J. Stadsnes, J. Bjordal, R. R. Vondrak, S. A Cummer, D. L. Chenette, G. K. Parks, M. J. Brittnacher and D. L. McKenzie, Global Scale Electron Precipitation Features seen in UV and X-rays During Substorms. J. Geophys. Res., Vol. 104, p. 10,191 - 10,204, 1999.

  • Dayside observations of energetic precipitation on the poleward edge of the cusp. Energetic electron precipitation are sometimes observed at very high latitudes on the dayside. One such event was studied by X-ray imaging from space in combination with riometer, allskycamera, photometers and magnetometers at the South Pole station.

    [pdf] N. Østgaard, D. L. Detrick, T. J. Rosenberg, R. R. Vondrak, H. U. Frey, S. B. Mende, S. E. Håland, J Stadsnes, High-latitude dayside energetic precipitation and IMF BZ rotations. J. Geophys. Res., Vol. 108 (A4), 8013, doi:10.1029/2002JA009350, 2003.

    Substorm scenarios

    In different collaborations we have studied possible onset triggering mechanisms and scenarios. The studies we have been involved in can be explained in the framework of the revised Near-Earth neutral line model, allthough conclusive evidence for a specific substorm model have not been found.

    [pdf] S. B. Mende, C. W. Carlson, H. U. Frey, L. M. Peticolas, N. Østgaard. FAST and IMAGE-FUV observations of a Substorm onset. J. Geophys. Res., Vol. 108, (A9), 1344, doi:10.1029/2002JA009787, 2003.
    [pdf] J. A. Slavin, D. H. Fairfield, R. P. Lepping, M. Hesse, A. Ieda, E. Tanskanen, N. Østgaard, T. Mukai, T. Nagai, H. J. Singer and P. R. Sutcliffe, Simultaneous observations of earthward flow bursts and plasmoid ejection during magnetospheric substorms. J. Geophys. Res.,Vol. 107, NO. A7, 10.1029/2000JA003501, 2002.
    [pdf] S. Håland, N. Østgaard, J. Bjordal, J.Stadsnes, S. Ullaland, G. D. Reeves, D. L. Chenette, B. Wilken, T. Yamamoto, T. Doke, G. K. Parks and M. J. Brittnacher, Magnetospheric and Ionospheric Response to a Substorm: Geotail HEP-LD and Polar PIXIE Observations, J. Geophys. Res., Vol. 104, No. A12, p. 28,459. 1999.

    Global and local pulsations of energetic electron precipitation

    Pulsations of energetic electron precipitation is usually considered to be postmidnight/morning sector phenomena. We have been involved in a few such studies and have some ongoing studies on this topic using X-ray imaging from space.

    [pdf] N. Østgaard, J. Stadsnes, K. Aarsnes, F. Søraas, Karl Måseide, M. Smith and J.Sharber, Simultaneous Measurements of X-rays and Electrons During a Pulsating Aurora. Ann. Geophysicae, 16, 148 - 160, 1998.

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