Solar cycle 24: An unusual polar field reversal

Abstract

Context. It is well known that the polarity of the Sun's magnetic field reverses or flips around the maximum of each 11 year solar cycle. This is commonly known as polar field reversal and plays a key role in deciding the polar field strength at the end of a cycle, which is crucial for the prediction of the upcoming cycle. Aims. To investigate solar polar fields during cycle 24, using measurements of solar magnetic fields in the latitude range 55 • -90 • and 78 • -90 • , to report a prolonged and unusual hemispheric asymmetry in the polar field reversal pattern in solar cycle 24. Methods. This study was carried out using medium resolution line-of-sight synoptic magnetograms from the magnetic database of the National Solar Observatory at Kitt Peak (NSO/KP), USA for the period between February 1975 and October 2017, covering solar cycles 21 -24 and high-resolution line-of-sight synoptic magnetograms from the Michaelson Doppler Imager instrument onboard the Solar Heliospheric Observatory. Synoptic magnetograms using radial measurements from the Heliospheric Magnetic Imager instrument onboard the Solar Dynamics Observatory, covering solar cycle 23 and 24, were also used. Results. We show that the Southern solar hemisphere unambiguously reversed polarity in mid-2013 while the reversal in the field in the Northern solar hemisphere started as early as June 2012, was followed by a sustained period of near-zero field strength lasting until the end of 2014, after which the field began to show a clear rise from its near-zero value. While this study compliments a similar study carried out using microwave brightness measurements (Gopalswamy et al. 2016) which claimed that the field reversal process in cycle 24 was completed by the end of 2015, our results show that the field reversal in cycle 24 was completed earlier i.e. in late 2014. Signatures of this unusual field reversal pattern were also clearly identifiable in the solar wind, using our observations of interplanetary scintillation at 327 MHz which supported our magnetic field observations and confirmed that the field reversal process was completed at the end of 2014.

Beyond the Minisolar Maximum of Solar Cycle 24: Declining Solar Magnetic Fields and the Response of the Terrestrial Magnetosphere

“…Hence, it is evident that the long-term study of the shape and location of the BS and MP is important. The other point to note here that the polar fields studied by different researchers are actually the signed values of solar polar fields (see Figure 1 in Janardhan et al (2018)), which reverse polarity around solar maximum of each solar cycle. In this study we therefore, attempt to quantify the changes in shape and location of the Earth's magnetosphere over the last four solar cycles and to compare the results obtained using both empirical and numerical models.…”

“…Our studies of solar photospheric magnetic fields (Bisoi, Janardhan, Chakrabarty, et al, 2014;Janardhan et al, 2010Janardhan et al, , 2015Janardhan et al, , 2018 in the past few years have shown a steady and continuous decline of solar high-latitude photospheric fields since mid-1990s. This link has been of particular interest to the solar and space science community due to the peculiar behavior seen in solar cycles 23 and 24 and in view of the long-term changes taking place on the Sun and in the solar wind over the past three solar cycles.…”

“…Recently, Samsonov et al (2019), using solar wind observations and empirical magnetopause models, also reported an increase in average annual magnetopause stand-off distance by nearly 2 R E between 1991 (9.7 R E ) and 2009 (11.6 R E ). It is seen (see Figure 3 in Janardhan et al (2015) and Figure 5 in Janardhan et al (2018)) that the unsigned values of solar magnetic fields in the latitude range 0 • -45 • follow the solar cycle with the fields attaining their maximum at around solar maximum of each solar cycle. , while the open blue circles are annual means shown with 1 error bars.…”

“…The other study that we found for such long-term trends for the MP stand-off distances is by McComas et al (2013), wherein the authors reported the canonical stand-off distance of the MP to be about 11 Earth radii (R E ), for the period 2009 to 2013, covering the minimum of cycle 23 to the early rise phase of cycle 24, compared to about 10 R E for the period 1974 to 1994, covering cycles 21-22, while the normal value of the MP is usually ∼10 R E . It must however, be kept in mind that different researchers have used different latitude ranges to estimate polar fields, namely, poleward of 45 • (Bisoi, Janardhan, Chakrabarty, et al, 2014;Janardhan et al, 2011), 55 • (Wilcox Solar Observatory polar fields, http://wso.stanford.edu/Polar.html, Janardhan et al (2018)), 60 • (Gopalswamy et al, 2012(Gopalswamy et al, , 2016Sun et al, 2015;de Toma, 2011), and70 • (Muñoz-Jaramillo et al, 2012). Recently, Samsonov et al (2019), using solar wind observations and empirical magnetopause models, also reported an increase in average annual magnetopause stand-off distance by nearly 2 R E between 1991 (9.7 R E ) and 2009 (11.6 R E ).…”

“…In earlier studies, we have also shown the steady decline in solar wind microturbulence levels in the inner heliosphere, spanning heliocentric distances from 0.2 to 0.8 AU (Bisoi, Janardhan, Ingale, et al, 2014;Janardhan et al, 2011Janardhan et al, , 2015, in sync with the decline in photospheric magnetic fields. The long-term declining trends seen in both photospheric magnetic fields and solar wind microturbulence levels over the entire inner heliosphere, coupled with the unusually deep solar minimum in cycle 23 and the very unusual solar polar field conditions seen in cycle 24 (Gopalswamy et al, 2016;Janardhan et al, 2018), implies that these changes could directly affect the terrestrial magnetosphere. IPS essentially provides one with an idea of the large-scale structure of the solar wind (Ananthakrishnan et al, 1995(Ananthakrishnan et al, , 1980.…”