Strong Changes in H+ Fraction and Upper Transition Height Induced by Minor Geomagnetic Storm of September 23-26, 2016

0477982 - ÚFA 2018 BE eng A - Abstrakt
Kotov, D. - Richards, P. - Erickson, P. J. - Goncharenko, L. - Bogomaz, O. - Truhlík, Vladimír - Shulha, M. - Siusiuk, M. - Domnin, I.
Strong Changes in H+ Fraction and Upper Transition Height Induced by Minor Geomagnetic Storm of September 23-26, 2016.
Papers from URSI: XXXIInd General Assembly and Scientific Symposium (URSI GASS). Ghent: URSI, 2017. G36-2.
[URSI General Assembly and Scientific Symposium (URSI GASS) 2017 /32./. 19.08.2017-26.08.2017, Montreal]
Institucionální podpora: RVO:68378289
Klíčová slova: geomagnetic storm * incoherent scatter (IS) radar * Field Line Interhemispheric Plasma (FLIP) model
Kód oboru RIV: DG - Vědy o atmosféře, meteorologie

The impact of the weak geomagnetic disturbances on the topside H+ ion fraction and upper transition height (UTH) is still poorly investigated. It was found that the nighttime UTH values over Kharkiv are quite sensitive to the weak changes in Ap index; the value of this sensitivity was estimated as ~ 4 km/nT. A factor of ~2.5 less sensitivity is seen from Arecibo ISR data. However, physical mechanisms to explain the observed UTH behaviour are not fully understood.
In this case study, we consider geomagnetic storm which began in the UT evening of September 24, 2016. The storm was very weak (minimal Dst value was -31 nT but typically the index was not less than -20 nT). However, spectacular decrease of H+ fraction was observed by Kharkiv IS radar. For instance, at the altitude of 550 km, diurnal maximum of H+ fraction was reducing by a factor of ~2.5 from the quiet night (from September 23 to 24) to the evening of September 25. Nighttime minimum of UTH also exhibits notable changes and was increasing from ~500 km to ~580 km during the same period. Estimated UTH sensitivity to Ap index is ~2.9 km/nT which is the same order of magnitude with the value obtained in [2].
Interesting result is that H+ fraction during the evening of September 25 is much closer to the prediction of TTS empirical model [3] than for the quiet night from September 23 to 24 when the model underestimates H+ fraction by a factor of 3. This point supports our previous idea on the reasons for the relatively low TTS H+ fractions: data, on which TTS model predictions for solar minimum are based, were collected during the period of 1974 when geomagnetic conditions were often the modest disturbed (mean month Ap indices were 15-25 nT).
We use Field Line Interhemispheric Plasma (FLIP) model to study the possible mechanisms for such strong impact of weak geomagnetic disturbances on the nighttime H+ ion fraction.
Trvalý link: http://hdl.handle.net/11104/0274212