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Zhao L.,University of Michigan | Landi E.,University of Michigan | Gibson S.E.,NCAR HAO
Astrophysical Journal | Year: 2013

Since the unusually prolonged and weak solar minimum between solar cycles 23 and 24 (2008-2010), the sunspot number is smaller and the overall morphology of the Sun's magnetic field is more complicated (i.e., less of a dipole component and more of a tilted current sheet) compared with the same minimum and ascending phases of the previous cycle. Nearly 13 yr after the last solar maximum (∼2000), the monthly sunspot number is currently only at half the highest value of the past cycle's maximum, whereas the polar magnetic field of the Sun is reversing (north pole first). These circumstances make it timely to consider alternatives to the sunspot number for tracking the Sun's magnetic cycle and measuring its complexity. In this study, we introduce two novel parameters, the standard deviation (SD) of the latitude of the heliospheric current sheet (HCS) and the integrated slope (SL) of the HCS, to evaluate the complexity of the Sun's magnetic field and track the solar cycle. SD and SL are obtained from the magnetic synoptic maps calculated by a potential field source surface model. We find that SD and SL are sensitive to the complexity of the HCS: (1) they have low values when the HCS is flat at solar minimum, and high values when the HCS is highly tilted at solar maximum; (2) they respond to the topology of the HCS differently, as a higher SD value indicates that a larger part of the HCS extends to higher latitude, while a higher SL value implies that the HCS is wavier; (3) they are good indicators of magnetically anomalous cycles. Based on the comparison between SD and SL with the normalized sunspot number in the most recent four solar cycles, we find that in 2011 the solar magnetic field had attained a similar complexity as compared to the previous maxima. In addition, in the ascending phase of cycle 24, SD and SL in the northern hemisphere were on the average much greater than in the southern hemisphere, indicating a more tilted and wavier HCS in the north than the south, associated with the early reversal of the polar magnetic field in the north relative to the south. © 2013. The American Astronomical Society. All rights reserved. Source


Kleint L.,University of Applied Sciences and Arts Northwestern Switzerland | Heinzel P.,Academy of Sciences of the Czech Republic | Judge P.,NCAR HAO | Krucker S.,University of Applied Sciences and Arts Northwestern Switzerland | Krucker S.,University of California at Berkeley
Astrophysical Journal | Year: 2016

Enhanced continuum brightness is observed in many flares ("white light flares"), yet it is still unclear which processes contribute to the emission. To understand the transport of energy needed to account for this emission, we must first identify both the emission processes and the emission source regions. Possibilities include heating in the chromosphere causing optically thin or thick emission from free-bound transitions of Hydrogen, and heating of the photosphere causing enhanced H- continuum brightness. To investigate these possibilities, we combine observations from Interface Region Imaging Spectrograph (IRIS), SDO/Helioseismic and Magnetic Imager, and the ground-based Facility Infrared Spectrometer instrument, covering wavelengths in the far-UV, near-UV (NUV), visible, and infrared during the X1 flare SOL20140329T17:48. Fits of blackbody spectra to infrared and visible wavelengths are reasonable, yielding radiation temperatures ∼6000-6300 K. The NUV emission, formed in the upper photosphere under undisturbed conditions, exceeds these simple fits during the flare, requiring extra emission from the Balmer continuum in the chromosphere. Thus, the continuum originates from enhanced radiation from photosphere (visible-IR) and chromosphere (NUV). From the standard thick-target flare model, we calculate the energy of the nonthermal electrons observed by Reuven Ramaty High Energy Solar Spectroscope Imager (RHESSI) and compare it to the energy radiated by the continuum emission. We find that the energy contained in most electrons >40 keV, or alternatively, of ∼10%-20% of electrons >20 keV is sufficient to explain the extra continuum emission of ∼(4-8)x1010 erg s-1 cm-2. Also, from the timing of the RHESSI HXR and the IRIS observations, we conclude that the NUV continuum is emitted nearly instantaneously when HXR emission is observed with a time difference of no more than 15 s. © 2016. The American Astronomical Society. All rights reserved.. Source


Lopez R.E.,University of Texas at Arlington | Bhattarai S.K.,University of Texas at Arlington | Bruntz R.,University of Texas at Arlington | Pham K.,University of Texas at Arlington | And 4 more authors.
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2012

In this paper we examine the role of dayside merging between the interplanetary magnetic field (IMF) and the geomagnetic field in the generation of the polar cap potential in the ionosphere during the Whole Heliospheric Interval using the Coupled Magnetosphere Ionosphere Thermosphere (CMIT) and Lyon-Fedder-Mobarry (LFM) global simulations of the geospace system from the Center for Integrated Space Weather Modeling (CISM). We isolate the portion of the total ionospheric potential due to the viscous interaction by simulating the interval with a zero IMF, but with the same solar wind plasma conditions. For southward IMF, the cross polar cap potential is the sum of the merging potential and the viscous potential, so we can determine the merging potential by subtracting the viscous potential from the total potential. From the dependence of the merging potential on southward IMF we calculate a geoeffective length of 5 R E. For northward IMF the situation is more complicated since the cross polar cap potential, defined as the peak to peak potential, will be almost always either the value of the viscous potential or of the merging potential, whichever is larger. We find that during periods of northward IMF the cross polar cap potential can be less than what the viscous interaction would produce with no IMF present. This means that the viscous interaction is weakened by the cycle of merging and reconnection for northward IMF. Our results also indicate that current representations of merging rates or electric fields are flawed in the manner in which they describe northward IMF. Typical representations simply produce a weak reconnection rate when the IMF is northward that adds to the viscous potential to create a cross polar cap potential that is larger than the viscous potential, whereas the effect of merging for northward IMF reduces the viscous interaction so that the cross polar cap potential for moderate northward IMF values is smaller than the value that would be expected from solar wind plasma conditions of the viscous potential in isolation. © 2012 Elsevier Ltd. Source


Zhao L.,NCAR HAO | Zhao L.,University of Michigan | Gibson S.E.,NCAR HAO | Fisk L.A.,University of Michigan
Journal of Geophysical Research: Space Physics | Year: 2013

We investigate the characteristics and solar origins of a subpopulation of the solar wind that possesses extreme values of proton flux. Ulysses observations including solar wind magnetic flux, proton flux, number density and velocity, and ionic composition are examined in this study. We find that the departures of solar wind proton flux from its constancy occur for time intervals leading up to and encompassing the past two solar minima, and the extreme-proton-flux wind possesses the following characteristics: (1) it generally originates from sources middle-distant from the Heliospheric Current Sheet (HCS); (2) it is associated with a broad range of velocities and electron temperatures but excludes very fast/cold wind; (3) it exhibits anticorrelation between electron temperature and proton velocity, as does the rest of the solar wind; (4) it has extreme proton density values relative to the rest of the solar wind; and (5) the extreme-high-proton-flux wind has radial component of open magnetic flux (Br) greater than the rest of the solar wind, and both extreme-high and extreme-low wind do not possess the lowest values of Br flux. Comparing with SOHO EIT 195 Å coronal images, we find the observed extreme-proton-flux wind has temporal and spatial coincidence with the appearance of low-latitude coronal holes present in the recent two solar minima; the magnetic field lines extrapolated by the Potential Field Source Surface model confirm there are coronal pseudostreamer structures involved. So we propose that these extreme-proton-flux winds can be associated with mid- to low-latitude coronal holes and "pseudostreamer" structures. Key Points Extreme-proton-flux solar wind was seen at middle latitudes in two solar minima. This wind has unusual properties compared with the rest of the wind. This wind can be associated with ©2013. American Geophysical Union. All Rights Reserved. Source


Gibson S.E.,NCAR HAO | de Toma G.,NCAR HAO | Emery B.,NCAR HAO | Riley P.,Predictive Science Inc. | And 8 more authors.
Solar Physics | Year: 2011

Throughout months of extremely low solar activity during the recent extended solar-cycle minimum, structural evolution continued to be observed from the Sun through the solar wind and to the Earth. In 2008, the presence of long-lived and large low-latitude coronal holes meant that geospace was periodically impacted by high-speed streams, even though solar irradiance, activity, and interplanetary magnetic fields had reached levels as low as, or lower than, observed in past minima. This time period, which includes the first Whole Heliosphere Interval (WHI 1: Carrington Rotation (CR) 2068), illustrates the effects of fast solar-wind streams on the Earth in an otherwise quiet heliosphere. By the end of 2008, sunspots and solar irradiance had reached their lowest levels for this minimum (e.g., WHI 2: CR 2078), and continued solar magnetic-flux evolution had led to a flattening of the heliospheric current sheet and the decay of the low-latitude coronal holes and associated Earth-intersecting high-speed solar-wind streams. As the new solar cycle slowly began, solar-wind and geospace observables stayed low or continued to decline, reaching very low levels by June - July 2009. At this point (e.g., WHI 3: CR 2085) the Sun-Earth system, taken as a whole, was at its quietest. In this article we present an overview of observations that span the period 2008 - 2009, with highlighted discussion of CRs 2068, 2078, and 2085. We show side-by-side observables from the Sun's interior through its surface and atmosphere, through the solar wind and heliosphere and to the Earth's space environment and upper atmosphere, and reference detailed studies of these various regimes within this topical issue and elsewhere. © 2011 The Author(s). Source

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