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16 February 2005 | A group of researchers led by Dr. Y. Zhang and L.J. Paxton at the John Hopkins Applied Research Laboratory at Laurel, Maryland have studied the occurrence of night-side detached auroras (NDAs) that often occur during intense space weather storms. The auroral activity appears in satellite images as an area of emissions that are detached and located equatorward of the main band of activity, as the example to the left illustrates. To observers on the ground, NDA often appear as auroral arcs similar in appearance to stable auroral red (SAR) arcs, but they are more narrow in structure. NDA typically varies in width from about 0.5 to 2.5 degrees of latitude.
For years, the source of detached nightsite auroras has been debated. Until recently, this activity was believed to be the result of electron precipitation into the high-altitude F-region (300 to 500 km) and heating-induced emissions. However, recent evidence is now fairly conclusive that the dominant form of emission responsible for this activity is due to proton precipitation.
Data from the DMSP spacecraft suggest that the responsible protons can be either hard (energetic) with energies of near 10 keV, or soft with energies less than 10 keV. The auroral activity is not dominantly caused by electron precipitation, which had been previously assumed. Instead, electron induced emissions play a more minor role.
The researchers found that detached auroral activity occurs most frequently during the recovery phase of intense geomagnetic storms where the associated ring current index (Dst) falls below approximately -130 nanoTeslas (nT). They have also been observed during the rapid growth phase of geomagnetic storms when the Dst index drops rapidly.
The most common time of day for the occurrence of detached forms of auroral activity is between the 7 pm and 3 am magnetic local time (MLT). This suggests that the source of the proton precipitation is the ring current itself, where the high populations of protons/ions exist. The proposed theory suggests that the protons/ions interact with the cold, dense plasma at the plasmapause. The interaction causes the protons/ions to become scattered, where they are then precipitated into the auroral ionosphere.
The lower latitude of this form of activity (equatorward of the main belt of auroral activity) was determined to be relatively proportional to the magnitude of the ring current index, which is not necessarily unexpected given that the entire auroral oval itself tends to shift equatorward as Dst becomes more negative.
The research was published in the Journal of Geophysical Research, volume 110 on February 2005 (doi:10.1029/2004JA010498, 2005) by the American Geophysical Union.
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