Richard R. Radick

Tracking a major interplanetary disturbance with SMEI

[1] We present the first clear observations of an Earth-directed interplanetary disturbance tracked by the Solar Mass Ejection Imager (SMEI). We find that this event can be related to two halo CMEs seen at the Sun about 2 days earlier, and which merged in transit to 1 AU. The disturbance was seen about 16 hours before it reached Earth, and caused a severe geomagnetic storm at the time which would have been predicted had SMEI been operating as a real-time monitor. It is concluded that SMEI is capable of giving many hours advance warning of the possible arrival of interplanetary disturbances.

Stellar Activity and Brightness Variations: A Glimpse at the Sun's History

Radiometric measurements during the past decade from the Solar Maximum Mission and Nimbus 7 satellites have shown that the total solar irradiance varies in step with the suns 11-year magnetic activity cycle. Stellar observations from the Lowell and Mount Wilson observatories now confirm and elaborate this discovery. These measurements show that older stars similar to the sun tend to become brighter as their magnetic activity level increases, just as the sun does during its 11-year activity cycle. Younger stars, however, tend to become fainter as their magnetic activity level increases. This contrasting behavior suggests that the balance between the competing phenomena that influence solar brightness variability has shifted during the suns lifetime.

A Survey of Chromospheric Activity in the Solar-Type Stars in the Open Cluster M67*

We present the results of a spectroscopic survey of the Ca II H and K core strengths in a sample of 60 solar-type stars that are members of the solar-age and solar-metallicity open cluster M67. We adopt the HK index, defined as the summed H+K core strengths in 1 A bandpasses centered on the H and K lines, respectively, as a measure of the chromospheric activity that is present. We compare the distribution of mean HK index values for the M67 solar-type stars with the variation of this index as measured for the Sun during the contemporary solar cycle. We find that the stellar distribution in our HK index is broader than that for the solar cycle. Approximately 17% of the M67 Sun-like stars exhibit average HK indices that are less than solar minimum. About 7%-12% are characterized by relatively high activity in excess of solar maximum values, while 72%-80% of the solar analogs exhibit Ca II H+K strengths within the range of the modern solar cycle. The ranges given reflect uncertainties in the most representative value of the maximum in the HK index to adopt for the solar cycle variations observed during the period AD 1976-2004. Thus, ~20%-30% of our homogeneous sample of Sun-like stars have mean chromospheric H+K strengths that are outside the range of the contemporary solar cycle. Any cycle-like variability that is present in the M67 solar-type stars appears to be characterized by periods greater than ~6 yr. Finally, we estimate a mean chromospheric age for M67 in the range of 3.8-4.3 Gyr.

Design and development of the Advanced Technology Solar Telescope (ATST)

High-resolution studies of the Suns magnetic fields are needed for a better understanding of solar magnetic fields and the fundamental processes responsible for solar variability. The generation of magnetic fields through dynamo processes, the amplification of fields through the interaction with plasma flows, and the destruction of fields are still poorly understood. There is still incomplete insight as to what physical mechanisms are responsible for heating the corona, what causes variations in the radiative output of the Sun, and what mechanisms trigger flares and coronal mass ejections. Progress in answering these critical questions requires study of the interaction of the magnetic field and convection with a resolution sufficient to observe scales fundamental to these processes. The 4m aperture Advanced Technology Solar Telescope (ATST) will be a unique scientific tool, with excellent angular resolution, a large wavelength range, and low scattered light. With its integrated adaptive optics, the ATST will achieve a spatial resolution nearly 10 times better than any existing solar telescope. Building a large aperture telescope for viewing the sun presents many challenges, some of the more difficult being Heat control and rejection Contamination and scattered light control Control of telescope and instrument polarization Site selection This talk will present a short summary of the scientific questions driving the ATST design, the design challenges faced by the ATST, and the current status of the developing design and siting considerations

Solar adaptive optics at the National Solar Observatory

The National Solar Observatory at Sacramento Peak is developing adaptive optics (AO) for solar astronomy. We are currently implementing a low order adaptive optics system that will correct approximately 20 Zernike modes. The system design permits future expansion future expansion to about 80 Zernike modes. We are using a correlating Shack-Hartmann wavefront sensor. Since a point source is generally not available on the sun, wavefront tilts have to be derived by applying correlation techniques to images of extended objects, such as granulation, probes and sunspots.

The magnetic, basal, and radiative-equilibrium components in Mount Wilson CA II H + K fluxes

Mount Wilson Ca II H + K flux measurements of cool dwarf stars are analyzed and compared with stellar Mg II h + k fluxes, variability amplitudes, rotation rates, and solar data. It is concluded that the Mount Wilson Ca II H + K fluxes comprise three principal parts: (1) a photospheric contribution in the line wings, (2) a basal chromospheric component that appears to be unrelated to stellar magnetic activity and is, therefore, possibly nonmagnetic in origin, and (3) a chromospheric component which is associated with magnetically active regions and the (quiet and active) network. The basal chromosphere appears to cover the entire surface of magnetically inactive stars. The basal Ca II H + K flux density for solar-type stars equals the average emission observed in the centers of solar supergranulation cells, where the magnetic flux density is small. 27 refs.

Confronting a solar irradiance reconstruction with solar and stellar data

Context. A recent paper by Shapiro and colleagues (2011, A&A, 529, A67) reconstructs spectral and total irradiance variations of the Sun during the holocene. Aims. In this note, we comment on why their methodology leads to large (0.5%) variations in the solar TSI on century-long time scales, in stark contrast to other reconstructions which have 22 yr) components of the irradiance reconstructions with secular changes in stellar photometric data that span 20 years or less, and find that the Sun, if varying with such large amplitudes, would still lie within the distribution of stellar photometric variations measured over a 10−20 year period. However, the stellar time series are individually too short to see if the reconstructed variations will remain consistent with stellar variations when observed for several decades more. Conclusions. By adopting model A, Shapiro et al. have over-estimated quiet-Sun irradiance variations by about a factor of two, based upon a re-analysis of sub-mm data from the James Clerk Maxwell telescope. But both estimates are within bounds set by current stellar data. It is therefore vital to continue accurate photometry of solar-like stars for at least another decade, to reveal secular and cyclic variations on multi-decadal time scales of direct interest to the Sun.

The activity, variability, and rotation of lower main-sequence members of the Coma star cluster

Abstract : High-precision, differential, Stromgren b, y photometric observations of nine members of the Coma star cluster, spectral types F3 V to K0 V, were made. Four G-type stars in this sample were all variable on both seasonal and year-to-year time scales, and the single K-type star also showed hints of variability. In contrast, four F-type stars were not detectably variable on either time scale. The variable stars tended to become slightly bluer as they brightened. The low-level photometric variability of Coma stars appears to resemble closely that observed among similar stars in the Hyades cluster. We also measured rotation periods for the four G-type stars from modulation present in our photometric data. The rotation periods of these stars and similar stars in the Hyades are comparable. The fact that the Coma and Hyades clusters are essentially indistinguishable in terms of their activity, variability, and rotational characteristics presents difficulties for claims that the photometric Hyades anomaly is a consequence of stellar activity. Reprints.

The Solar Mass Ejection Imager (SMEI): A new tool for space weather

Later this year, the U.S.Air Force will launch the Solar Mass Ejection Imager (SMEI). SMEI is an instrument that will detect and measure coronal mass ejections (CMEs) which can cause large geomagnetic storms, a major component of space weather. CMEs are very large structures containing plasma and magnetic fields that are expelled from the Sun into the heliosphere at speeds of several hundred to over 1000 km s−1. CMEs often drive interplanetary shock waves which, upon arrival at Earth, can cause geomagnetic disturbances. There is currently no reliable way to accurately predict arrival of these disturbances at Earth or to study them in the inner heliosphere. SMEI is designed to fill this gap by detecting and tracking CMEs in interplanetary space before they reach Earth. SMEI data will be complementary to many other satellite missions and national programs, such as the current National Space Weather and International Solar Terrestrial Programs.

Coronal activity-rotation relations for lower main-sequence stars

The relationship between stellar soft X-ray emission and rotation has been examined using a sample of 157 lower main-sequence stars from various clusters and the field. The highest correlation between X-ray emission and rotation is found when the X-ray emission is normalized by the bolometric emission and the rotation by the Rossby number, the rotation period divided by the convective turnover time at the base of the convective zone. No statistically significant variation of the slope of the activity-rotation relation with B-V color is found for longer period stars. The early-to-mid F stars (B-V = 0.3-0.45) follow the general activity-rotation relation defined by later-type stars. 37 refs.