Solar Wind Influences on Properties of the Ionosphere
The Sun’s corona expands outward, populating the solar system with plasma. This plasma is known as the solar wind. The solar wind carries with it the Sun’s magnetic ﬁeld, which is also known as the interplanetary magnetic ﬁeld (IMF). The resulting conﬁguration of the IMF creates a current sheet at solar equatorial latitudes, which the Earth crosses as it orbits the Sun. When the Earth is on one side of the current sheet it is in a sector where the IMF is directed largely away from or toward the Sun. On the other side of the current sheet the IMF is in opposite direction. The crossing of the current sheet is known as a sector boundary crossing (SBC). The solar wind and IMF properties change signiﬁcantly near the current sheet, and this aﬀects the Earth’s ionosphere. The Super Dual Auroral Radar Network (SuperDARN) high frequency (HF) radar data rates from 2001-2011 were examined using several techniques: a superposed epoch analysis, a fast fourier transform (FFT) analysis, and a cross–correlation analysis. Data from multiple instruments were analyzed in this study. These include the solar wind and IMF data from spacecraft, observations of charged particles precipitating into the Earth’s ionosphere, echoes from ground–based SuperDARN radars, and data from gound–based neutron monitors that detect galactic cosmic rays. Solar wind and IMF properties change signiﬁcantly across a sector boundary. An increase in the IMF magnitude of about 30% occurs on the day of the SBC, and the IMF returns to pre–crossing values over the next two days. There is a decrease in the solar wind speed of about 15% the day before and the day of the SBC, and the solar wind density doubles at the time of the SBC. The polarity of the SBC does not appear to aﬀect the solar wind and IMF. A peak in the data rate of SuperDARN echoes from both the ionosphere and ground occurs within one day of the SBC, though the variability of these data is quite large. The hemispherical power, which is an estimation of the electron energy ﬂux precipitating into the ionosphere derived from satellite observations, increases following a SBC. Satellite particle data also revealed that the equatorward auroral oval boundary moves equatorward following a SBC. The cosmic ray counts at the Earth’s surface appear to be unaﬀected by the SBC. The solar wind and ionosphere data sets exhibited strong periodicities, and these were harmonics of the synodic rotational period of the Sun (approximately 27 days). Common periodicities observed were 27 days, 13.5 days, 9 days, 6.75 days and 5.4 days. There was a dominant 9–day periodicity observed in the solar wind and ionospheric data from 2005–2008, but was not observed in the solar 10.7 cm wavelength electromagnetic ﬂux. The 9-day periodicity in the solar wind during this period has been linked to three persistent features on the Sun that produced corotating high–speed streams, or areas of fast solar wind. The parameters whose change did not depend on the polarity of the SBC had periodicities that were half that of the SBCs. From the cross–correlation analysis some relationships between the data sets became evident. For periods of high solar wind speed there were low SuperDARN data rates, and vice versa. The solar wind speed and hemispherical power were found to be well correlated, while the hemispherical power and the SuperDARN scatter occurrence were found to be anticorrelated. The solar wind changes appear to be aﬀecting the state of the ionosphere, likely through particle precipitation. The SuperDARN scatter occurrence has been shown in past studies to be most greatly aﬀected by changes in the electron density proﬁle of the ionosphere, which can be inﬂuenced by changes in particle precipitation. These results demonstrate a link between the solar wind and the state of the ionosphere.
sector boundary, solar wind, ionosphere, SuperDARN, particle precipitation, cross correlation, spectral analysis
Master of Science (M.Sc.)
Physics and Engineering Physics