Acres Green, CO, United States
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Zurbuchen T.H.,University of Michigan | Von Steiger R.,International Space Science Institute | Von Steiger R.,University of Bern | Gruesbeck J.,University of Michigan | And 5 more authors.
Space Science Reviews | Year: 2012

In this discussion of observational constraints on the source regions and acceleration processes of solar wind, we will focus on the ionic composition of the solar wind and the distribution of charge states of heavy elements such as oxygen and iron. We first focus on the now well-known bi-modal nature of solar wind, which dominates the heliosphere at solar minimum: Compositionally cool solar wind from polar coronal holes over-expands, filling a much larger solid angle than the coronal holes on the Sun. We use a series of remote and in-situ characteristics to derive a global geometric expansion factor of ∼5. Slower, streamer-associated wind is located near the heliospheric current sheet with a width of 10-20°, but in a well-defined band with a geometrically small transition width. We then compute charge states under the assumption of thermal electron distributions and temperature, velocity, and density profiles predicted by a recent solar wind model, and conclude that the solar wind originates from a hot source at around 1 million K, characteristic of the closed corona. © 2012 The Author(s).

Emery B.A.,080 Center Green | Richardson I.G.,University of Maryland University College | Evans D.S.,National Oceanic and Atmospheric Administration | Rich F.J.,Massachusetts Institute of Technology | Wilson G.R.,U.S. Air force
Solar Physics | Year: 2011

The behavior of a number of solar wind, radiation belt, auroral and geomagnetic parameters is examined during the recent extended solar minimum and previous solar cycles, covering the period from January 1972 to July 2010. This period includes most of the solar minimum between Cycles 23 and 24, which was more extended than recent solar minima, with historically low values of most of these parameters in 2009. Solar rotational periodicities from 5 to 27 days were found from daily averages over 81 days for the parameters. There were very strong 9-day periodicities in many variables in 2005 - 2008, triggered by recurring corotating high-speed streams (HSS). All rotational amplitudes were relatively large in the descending and early minimum phases of the solar cycle, when HSS are the predominant solar wind structures. There were minima in the amplitudes of all solar rotational periodicities near the end of each solar minimum, as well as at the start of the reversal of the solar magnetic field polarity at solar maximum (~ 1980, ~ 1990, and ~ 2001) when the occurrence frequency of HSS is relatively low. Semiannual equinoctial periodicities, which were relatively strong in the 1995 - 1997 solar minimum, were found to be primarily the result of the changing amplitudes of the 13.5- and 27-day periodicities, where 13.5-day amplitudes were better correlated with heliospheric daily observations and 27-day amplitudes correlated better with Earth-based daily observations. The equinoctial rotational amplitudes of the Earth-based parameters were probably enhanced by a combination of the Russell-McPherron effect and a reduction in the solar wind-magnetosphere coupling efficiency during solstices. The rotational amplitudes were cross-correlated with each other, where the 27-day amplitudes showed some of the weakest cross-correlations. The rotational amplitudes of the > 2 MeV radiation belt electron number fluxes were progressively weaker from 27- to 5-day periods, showing that processes in the magnetosphere act as a low-pass filter between the solar wind and the radiation belt. The A p/K p magnetic currents observed at subauroral latitudes are sensitive to proton auroral precipitation, especially for 9-day and shorter periods, while the A p/K p currents are governed by electron auroral precipitation for 13.5- and 27-day periodicities. © 2011 Springer Science+Business Media B.V.

De Toma G.,080 Center Green | Gibson S.,080 Center Green | Emery B.,080 Center Green | Kozyra J.,University of Michigan
AIP Conference Proceedings | Year: 2010

We are currently observing the minimum phase of Cycle 23. Magnetic activity during the years 2006-2009 has been very weak with sunspot numbers reaching the lowest values in about 100 years. This long and extended minimum is characterized by weak polar magnetic fields, small polar coronal holes, and a relatively complex coronal morphology. This magnetic configuration at the Sun is remarkably different from the one observed during the previous two solar minima. We review observations made at the Sun and in the solar wind during the recent solar minima and discuss the implications of the observed differences for the heliosphere and geospace. © 2010 American Institute of Physics.

We analyze coronal holes present on the Sun during the extended minimum between Cycles 23 and 24, study their evolution, examine the consequences for the solar wind speed near the Earth, and compare it with the previous minimum in 1996. We identify coronal holes and determine their size and location using a combination of EUV observations from SOHO/EIT and STEREO/EUVI and magnetograms. We find that the long period of low solar activity from 2006 to 2009 was characterized by weak polar magnetic fields and polar coronal holes smaller than observed during the previous minimum. We also find that large, low-latitude coronal holes were present on the Sun until 2008 and remained important sources of recurrent high-speed solar wind streams. By the end of 2008, these low-latitude coronal holes started to close down, and finally disappeared in 2009, while smaller, mid-latitude coronal holes formed in the remnants of Cycle 24 active regions shifting the sources of the solar wind at the Earth to higher latitudes. © 2010 Springer Science+Business Media B.V.

Zhao L.,University of Michigan | Zhao L.,080 Center Green | Fisk L.,University of Michigan
Solar Physics | Year: 2011

The properties of the heliospheric magnetic field and the solar wind were substantially different in the unusual solar minimum between Cycles 23 and 24: the magnetic-field strength was substantially reduced, as were the flow properties of the solar wind, such as the mass flux. Explanations for these changes are offered that do not require any substantial reconsideration of the general understandings of the behavior of the heliospheric magnetic field and the solar wind that were developed in the minimum of Cycle 22 - 23. Solar-wind composition data are used to demonstrate that there are two distinct regions of solar wind: solar wind likely to originate from the stalk of the streamer belt (the highly elongated loops that underlie the heliospheric current sheet), and solar wind from outside this region. The region outside the streamer-stalk region is noticeably larger in the minimum of Cycle 23 - 24; however, the increased area can account for the reduction in the heliospheric magnetic-field strength in this minimum. Thus, the total magnetic flux contained in this region is the same in the two minima. Various correlations among the solar-wind mass flux and coronal electron temperature inferred from solar-wind charge states were developed for the Cycle 22 - 23 solar minimum. The data for the minimum of Cycle 23 - 24 suggest that the correlations still hold, and thus the basic acceleration mechanism is unchanged in this minimum. © 2011 Springer Science+Business Media B.V.

Solar cycle 23 behaved differently than cycle 22 in many ways. Certain properties, namely the long minimum at the end of cycle 23, weakening of polar fields, shrinking of polar coronal holes, reduction in the terrestrial atmospheric neutral density layer, have been identified as unusual compared to several past cycles. The origin of these differences most likely lies in the ways the dynamo has operated that led to distinctly different generation and evolution of the large-scale magnetic fields in cycles 22 and 23. Certain differences in the properties of Galactic-Cosmic Rays during cycles 22 and 23 have recently been explained by the differences in evolutionary pattern of coronal holes, which are linked to the dynamo-generated large-scale magnetic fields. In this paper, I will discuss the differences in the solar interior dynamics, particularly the properties of flow fields and their influence in governing the evolution of dynamo-generated magnetic fields during cycles 22 and 23, respectively. © 2011 The Author(s).

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