Hans-R. Müller

Dartmouth College, Hanover, New Hampshire, United States

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Publications (5)7.47 Total impact

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    ABSTRACT: Discovery of the Van Allen radiation belts by instrumentation flown on Explorer 1 in 1958 was the first major discovery of the Space Age. A view of the belts as distinct inner and outer zones of energetic particles with different sources was modified by observations made during the Cycle 22 maximum in solar activity in 1989–1991, the first approaching the activity level of the International Geophysical Year of 1957–1958. The dynamic variability of outer zone electrons was measured by the NASA–Air Force Combined Radiation Release and Effects Satellite launched in July 1990. This variability is caused by distinct types of heliospheric structure which vary with the solar cycle. The largest fluxes averaged over a solar rotation occur during the declining phase from solar maximum, when high-speed streams and co-rotating interaction regions (CIRs) dominate the inner heliosphere, leading to recurrent storms. Intense episodic events driven by high-speed interplanetary shocks launched by coronal mass ejections (CMEs) prevail around solar maximum when CMEs occur most frequently. Only about half of moderate storms, defined by intensity of the ring current, lead to an overall flux increase, emphasizing the need to quantify loss as well as source processes; both increase when the magnetosphere is strongly driven. Three distinct types of acceleration are described in this review: prompt and diffusive radial transport, which increases energy while conserving the first invariant, and local acceleration by waves, which change the first invariant. The latter also produce pitch angle diffusion and loss, as does outward radial transport, especially when the magnetosphere is compressed. The effect of a dynamic magnetosphere boundary on radiation belt electrons is described in the context of MHD-test particle simulations driven by measured solar wind input.
    Journal of Atmospheric and Solar-Terrestrial Physics 03/2008; 70(5-70):708-729. DOI:10.1016/j.jastp.2007.11.003 · 1.47 Impact Factor
  • Hans-R. Müller · Laura M. Woodman · Gary P. Zank ·
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    ABSTRACT: A suite of 64 global heliospheric models, for which the interstellar densities and temperatures are varied within reasonable bounds, is analyzed with respect to the location of the termination shock on and off the stagnation axis, its temperature, and its compression ratio. The empirical relations regarding the termination shock, the heliopause and the interstellar bow shock, are discussed, as are the physical reasons behind these relations. © 2008 American Institute of Physics heliospheric termination shock.
    01/2008; 1039. DOI:10.1063/1.2982475
  • Gary Zank · Hans-R. Müller · Vladimir Florinski · Priscilla Frisch ·
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    ABSTRACT: While unchanging on human timescales, the interstellar environment has been and will be very different from current conditions. Historically, the heliosphere and solar system have been in regions of the galaxy that were much hotter, or contained higher concentrations of neutral hydrogen, or with interstellar clouds with higher or lower speeds, than is the case today. In this chapter, we describe the response of the heliosphere to different interstellar environments, taking into account the basic physics of the coupled neutral hydrogen and plasma selfconsistently.
    09/2006: pages 23-51;
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    ABSTRACT: A suite of global heliospheric models, for which the interstellar densities and temperatures are varied within reasonable bounds, is analyzed with respect to the heliospheric morphology and the locations of heliospheric boundaries, to arrive at empirical relations and to assess the sensitivity of the heliosphere to the interstellar conditions. Additionally, the differences between the termination shock at the Voyager 1 location and on the stagnation axis are discussed.
    09/2006; 858. DOI:10.1063/1.2359302
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    ABSTRACT: At present, the heliosphere is embedded in a warm low density interstellar cloud that belongs to a cloud system flowing through the local standard of rest with a velocity near ~18 km/s. The velocity structure of the nearest interstellar material (ISM), combined with theoretical models of the local interstellar cloud (LIC), suggest that the Sun passes through cloudlets on timescales of < 10^3 - 10^4 yr, so the heliosphere has been, and will be, exposed to different interstellar environments over time. By means of a multi-fluid model that treats plasma and neutral hydrogen self-consistently, the interaction of the solar wind with a variety of partially ionized ISM is investigated, with the focus on low density cloudlets such as are currently near the Sun. Under the assumption that the basic solar wind parameters remain/were as they are today, a range of ISM parameters (from cold neutral to hot ionized, with various densities and velocities) is considered. In response to different interstellar boundary conditions, the heliospheric size and structure change, as does the abundance of interstellar and secondary neutrals in the inner heliosphere, and the cosmic ray level in the vicinity of Earth. Some empirical relations between interstellar parameters and heliospheric boundary locations, as well as neutral densities, are extracted from the models.
    The Astrophysical Journal 07/2006; 647(2). DOI:10.1086/505588 · 5.99 Impact Factor