Solar and Space Physics A Science for a Technological Society (2013) / Chapter Skim
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2 Solar and Space Physics: Recent Discoveries, Future Frontiers
2 Solar and Space Physics: Recent Discoveries, Future Frontiers
Pages 38-66
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From page 38... ...
In contrast with the EUV radiation, the solar wind does not impact Earth directly but instead encounters Earth's dipolar magnetic field, which deflects the solar wind and channels electric currents and energetic particles to the polar regions, shielding the middle and equatorial atmosphere. Earth is therefore best understood not as orbiting the Sun in isolation through a vacuum, but as a physical system intimately linked to the highly variable solar atmosphere that engulfs the entire solar system.
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From page 39... ...
solar wind intensify the Van Allen radiation belts, drive the aurora and powerful electric currents on Earth, and violently churn the ionosphere and uppermost atmosphere. There is a growing appreciation that solar systems are commonplace in the universe and that the physical processes active in Earth's heliosphere are universal.
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From page 40... ...
Researchers are now poised to answer questions concerning universal physical processes, advance understanding of the complex coupling and nonlinear dynamics of the heliosphere, and apply this understanding for mitigation of harmful impacts to Earth's technological infrastructures. To show how the recommendations of this report follow from the flow of scientific discovery, a selection of the most salient discoveries and advances are presented below.1 The Sun and Heliosphere The Solar Dynamo and Activity Over the past decade, the solar dynamo, which is the source of the Sun's magnetic field and the resultant dissipation that drives solar activity, continued as a high-priority focus of research.
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From page 41... ...
This enhanced galactic cosmic-ray flux was caused by reduced modulation in a historically weak solar wind with slower speeds, lower magnetic field, and historically low activity. The extended solar minimum, prolonged period of low sunspot numbers, and record cosmic-ray intensity led to suggestions that the Sun might be entering an extended period of minimum activity such as that observed (in sunspot, 14C and 10Be data)
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From page 42... ...
Rempel, Numerical sunspot models: Robustness of photospheric velocity and magnetic field structure, Astrophysical Journal 750(1)
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From page 43... ...
Temperature anisotropies with respect to the local magnetic field of solar wind H+ and He2+ were shown to be limited by the mirror and firehose instabilities.2 These observations constrain the possible mechanisms of solar wind heating. Scientists have also discovered that magnetic reconnection between adjacent domains of opposing magnetic fields is ubiquitous in the solar wind but appears to involve little particle acceleration near heliospheric reconnection sites -- a surprise, given the 2 See J.C.
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From page 44... ...
Energetic neutral atom (ENA) maps by IBEX and Cassini show an unpredicted "ribbon" of emissions from the outer heliosphere, apparently ordered by the local interstellar magnetic field (Figure 2.3)
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From page 45... ...
None of the earlier models pre Figure 2-3 combined dicted the ribbon or belt. SOURCE: Courtesy of the Interstellar Boundary Explorer Mission Team.
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From page 46... ...
The magnetospheric equatorial ring current is enhanced during geomagnetic storms, and it perturbs the strength of the magnetic field at Earth's surface. Understanding its dynamics is crucial for establishing a predictive capability of the response of geospace to storms.
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From page 47... ...
These ideas led to predictions that facilitated the first direct detection of the ion diffusion region (where the ions decouple from the magnetic field) in the magnetosphere and in the laboratory, as well as glimpses of the much smaller electron diffusion region (where the electrons decouple from the magnetic field)
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From page 48... ...
The results have been incorporated into time-dependent models of the radiation belts and the ring current. Scientists now know that storm-time particle dynamics are the result
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From page 49... ...
Colors in the radiation belts indicate relative number flux. The auroral zone colors reflect precipitation to the atmosphere.
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From page 50... ...
Spacecraft observations and numerical simulations reveal that solar-wind plasma entry into the magnetosphere is surprisingly efficient under "quiescent" conditions of a northward interplanetary magnetic field. This plasma in turn participates as a substantial element of the storm-time ring-current development when southward interplanetary magnetic fields couple with and energize the magnetosphere.
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From page 51... ...
Perhaps not surprisingly, several scientific discoveries involve physical processes that extend across the regions of interest of these agencies, and across nations. Active Ionosphere During Solar Minimum Gradual changes in solar activity, solar wind, solar EUV radiation, and Earth's magnetic field play a significant role in defining the long-term variation in the geospace environment.
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From page 52... ...
MacDowall, and A Lecacheux, The reversal of the rotational modulation rates of the north and south components of Saturn kilometric radiation near equinox, Figure 2-7 and 9-7 Geophysical Research Letters 37:L24101, doi:10.1029/2010GL045796, 2010.
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From page 53... ...
The solar wind-magnetosphere interaction on the dayside, that is, magnetopause reconnection, is a copious source of electromagnetic energy that propagates along the magnetic field into the ionosphere at high latitudes near noon. This energy is converted to heat and momentum through ion-neutral interactions and promotes resonant heating of O+ that drives outflows.
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From page 54... ...
Current estimates indicate that waves propagating upward from the lower atmosphere contribute about as much to the energy transfer in the IT system as does forcing from above in the forms of solar EUV and UV radiation, precipitating particles, resistive heating, and winds driven by magnetospheric convection. Thermospheric Climate Change A systematic decrease by several percent per decade in thermosphere mass density is now evident in the record of satellite orbit decay measured since the beginning of the space age.
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From page 55... ...
Indeed, evidence for decadal-scale periodicities has been found in the luminosities of other Sun-like stars. Scientists know that the twisting and amplification of seed magnetic fields in the Sun's convective zone, the outer one-third of the Sun, are the source of the solar magnetic field.
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From page 56... ...
The magnetic energy accumulates over days, weeks, or perhaps longer. When adjacent magnetic fields pointing in opposite directions become sufficiently strong, the magnetic fields explosively annihilate each other during magnetic reconnection (see Figure 1.3)
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From page 57... ...
Discover how the Sun interacts with the local interstellar medium. Challenges Related to Solar Wind-Magnetosphere Interactions While the broad view of how reconnection takes place and drives convection in the magnetosphere is now well established, the underlying physics of magnetic reconnection in the collisionless regime of the magnetosphere is not yet understood well enough to enable prediction of when, where, and how fast this process will occur and how it contributes to mass, energy, and momentum transport.
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From page 58... ...
The acceleration of particles in the radiation belts is believed to arise from a combination of compression as particles move from the weak magnetic field region in the distant magnetotail into the region of high magnetic field near Earth and the interaction with intense waves generated in the radiation belts themselves. NASA's Radiation Belts Storm Probes (RBSP; renamed the Van Allen Probes)
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From page 59... ...
Blake, Relationship of the Van Allen radiation belts to solar wind drivers, Journal of Atmospheric and Solar-Terrestrial Physics 70(5) :708-729, 2008, copyright 2008, with permission from Elsevier.
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From page 60... ...
The left-hand panels show only H+, the only species in the simulation, whereas the right-hand panels show the ionospheric O+, which is added to the H+. The red lines in each panel show magnetic field lines in the region of interest.
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From page 61... ...
Although planetary systems exhibit analogous structures, the contrasting dynamics, boundary conditions, and magnetic fields make their detailed study of unique importance for testing theories and models. Jupiter's moon Io, deep within the enormous Jovian magnetosphere, is a copious source of neutral gas, which, upon ionization, is a dominant drag force on the rapidly co-rotating magnetic field of the planet.
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From page 62... ...
The pathways through which ions and neutrals interact are of course fundamental to space physics, given that they occur at all planets with atmospheres, at comets, and within the magnetospheres of Jupiter and Saturn. For example, in Earth's ionosphere at an altitude from 100 to 130 km the collisions between ions and electrons and neutrals enable current to flow across the local magnetic field, which facilitates closure of currents flowing along magnetic fields from the magnetosphere.
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From page 63... ...
SOLAR AND SPACE PHYSICS: RECENT DISCOVERIES, FUTURE FRONTIERS 63 FIGURE 2.11 Top: Mean 1984-2009 January daytime convective cloud amount in percentage from ISCCP-D2. Blue indicates 10-15 percent, yellow/green indicates approximately 8 percent, and red indicates 0-4 percent.
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From page 64... ...
This well-understood role of CO2 as an effective radiator of energy in the upper atmosphere has produced a systematic decrease in thermospheric mass density by several percent per decade near the 400-km altitude. This systematic decrease follows from the record of satellite orbit decay measured since the beginning of the space age (Figure 2.13)
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From page 65... ...
Figure 2-13 and 8-18 vector editable, may be enlarged RISING TO THE CHALLENGES OF THE COMING DECADE Achievement of the survey committee's four key science goals (see Chapter 1) for the coming decade requires addressing the 12 science challenges, discussed above and also listed in Table 2.1, for the three subdisciplines of solar and space physics.
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From page 66... ...
Finally, the nearly explosive growth in the ability to model complex phenomena in solar and space physics with realistic numerical simulations suggests that the field is on the cusp of greatly expanded predictive power and fundamental understanding. The advanced state of theory and simulation also provides a powerful opportunity to couple efforts in this area with observations, which will always remain limited in key aspects, to realize the full potential of the observations and their implications for understanding the underlying physical processes that they reflect.
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