R. A. Howard

Bio: R. A. Howard is an academic researcher from United States Naval Research Laboratory. The author has contributed to research in topics: Coronal mass ejection & Corona. The author has an hindex of 32, co-authored 100 publications receiving 5023 citations.

The Large Angle Spectroscopic Coronagraph (LASCO)

Abstract: The Large Angle Spectroscopic Coronagraph (LASCO) is a three coronagraph package which has been jointly developed for the Solar and Heliospheric Observatory (SOHO) mission by the Naval Research Laboratory (USA), the Laboratoire d’Astronomie Spatiale (France), the Max-Planck-Institut fur Aeronomie (Germany), and the University of Birmingham (UK) LASCO comprises three coronagraphs, C1, C2, and C3, that together image the solar corona from 11 to 30 R⊙ (C1: 11–3 R⊙, C2: 15–6 R⊙, and C3: 37 – 30 R⊙) The C1 coronagraph is a newly developed mirror version of the classic internally-occulted Lyot coronagraph, while the C2 and C3 coronagraphs are externally occulted instruments High-resolution imaging spectroscopy of the corona from 11 to 3 R⊙ can be performed with the Fabry-Perot interferometer in C1 High-volume memories and a high-speed microprocessor enable extensive on-board image processing Image compression by a factor of about 10 will result in the transmission of 10 full images per hour

The SOHO/LASCO CME Catalog

Abstract: Coronal mass ejections (CMEs) are routinely identified in the images of the solar corona obtained by the Solar and Heliospheric Observatory (SOHO) mission’s Large Angle and Spectrometric Coronagraph (LASCO) since 1996. The identified CMEs are measured and their basic attributes are cataloged in a data base known as the SOHO/LASCO CME Catalog. The Catalog also contains digital data, movies, and plots for each CME, so detailed scientific investigations can be performed on CMEs and the related phenomena such as flares, radio bursts, solar energetic particle events, and geomagnetic storms. This paper provides a brief description of the Catalog and summarizes the statistical properties of CMEs obtained using the Catalog. Data products relevant to space weather research and some CME issues that can be addressed using the Catalog are discussed. The URL of the Catalog is: http://cdaw.gsfc.nasa.gov/CME_list.

The dynamical nature of coronal streamers

Abstract: Recent high-sensitivity imaging of the Sun's white-light corona from space has revealed a variety of unexpected small-scale phenomena, including plasma blobs that are ejected continually from the cusplike bases of streamers, fine raylike structures pervading the outer streamer belt, and inflows that occur mainly during times of high solar activity. These phenomena can be interpreted as different manifestations of magnetic field line reconnection, in which plasma and magnetic flux are exchanged between closed and open field regions of the corona. The observations provide new insights into a number of long-standing questions, including the origin of the streamer material in the outer corona, the sources of the slow solar wind, and the mechanisms that regulate the interplanetary magnetic field strength.

Associations between coronal mass ejections and soft X-ray events

Abstract: The association between white light coronal mass ejections (CMEs) and full-disk X ray events have been examined as a function of X ray duration during the recent years of high sunspot activity (1979-1981). On a time scale of hours, no duration interval has been found that separates X ray events into two distinct classes depending on whether or not they have associated CMEs. Rather, the tendency for long-duration X ray events to have associated CMEs reflects the fact that, as X ray duration increases, the differential distribution of events without CMEs falls off faster than the distriution of X ray events with CMEs.

Direct Detection of a CME-Associated Shock in LASCO White Light Images

Abstract: The LASCO C2 and C3 coronagraphs recorded a unique coronal mass ejection on April 2, 1999. The event did not have the typical three-part CME structure and involved a small filament eruption without any visibile overlying streamer ejecta. The event exhibited an unusually clear signature of a wave propagating at the CME flanks. The speed and density of the CME front and flanks were consistent with the existence of a shock. To better establish the nature of the white light wave signature, we employed a simple MHD simulation using the LASCO measurements as constraints. Both the measurements and the simulation strongly suggest that the white light feature is the density enhancement from a fast-mode MHD shock. In addition, the LASCO images clearly show streamers being deflected when the shock impinges on them. It is the first direct imaging of this interaction.

Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI)

Abstract: The Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) is a five telescope package, which has been developed for the Solar Terrestrial Relation Observatory (STEREO) mission by the Naval Research Laboratory (USA), the Lockheed Solar and Astrophysics Laboratory (USA), the Goddard Space Flight Center (USA), the University of Birmingham (UK), the Rutherford Appleton Laboratory (UK), the Max Planck Institute for Solar System Research (Germany), the Centre Spatiale de Leige (Belgium), the Institut d’Optique (France) and the Institut d’Astrophysique Spatiale (France). SECCHI comprises five telescopes, which together image the solar corona from the solar disk to beyond 1 AU. These telescopes are: an extreme ultraviolet imager (EUVI: 1–1.7 R⊙), two traditional Lyot coronagraphs (COR1: 1.5–4 R⊙ and COR2: 2.5–15 R⊙) and two new designs of heliospheric imagers (HI-1: 15–84 R⊙ and HI-2: 66–318 R⊙). All the instruments use 2048×2048 pixel CCD arrays in a backside-in mode. The EUVI backside surface has been specially processed for EUV sensitivity, while the others have an anti-reflection coating applied. A multi-tasking operating system, running on a PowerPC CPU, receives commands from the spacecraft, controls the instrument operations, acquires the images and compresses them for downlink through the main science channel (at compression factors typically up to 20×) and also through a low bandwidth channel to be used for space weather forecasting (at compression factors up to 200×). An image compression factor of about 10× enable the collection of images at the rate of about one every 2–3 minutes. Identical instruments, except for different sizes of occulters, are included on the STEREO-A and STEREO-B spacecraft.

The STEREO Mission: An Introduction

Abstract: The twin STEREO spacecraft were launched on October 26, 2006, at 00:52 UT from Kennedy Space Center aboard a Delta 7925 launch vehicle. After a series of highly eccentric Earth orbits with apogees beyond the moon, each spacecraft used close flybys of the moon to escape into orbits about the Sun near 1 AU. Once in heliospheric orbit, one spacecraft trails Earth while the other leads. As viewed from the Sun, the two spacecraft separate at approximately 44 to 45 degrees per year. The purposes of the STEREO Mission are to understand the causes and mechanisms of coronal mass ejection (CME) initiation and to follow the propagation of CMEs through the inner heliosphere to Earth. Researchers will use STEREO measurements to study the mechanisms and sites of energetic particle acceleration and to develop three-dimensional (3-D) time-dependent models of the magnetic topology, temperature, density and velocity of the solar wind between the Sun and Earth. To accomplish these goals, each STEREO spacecraft is equipped with an almost identical set of optical, radio and in situ particles and fields instruments provided by U.S. and European investigators. The SECCHI suite of instruments includes two white light coronagraphs, an extreme ultraviolet imager and two heliospheric white light imagers which track CMEs out to 1 AU. The IMPACT suite of instruments measures in situ solar wind electrons, energetic electrons, protons and heavier ions. IMPACT also includes a magnetometer to measure the in situ magnetic field strength and direction. The PLASTIC instrument measures the composition of heavy ions in the ambient plasma as well as protons and alpha particles. The S/WAVES instrument uses radio waves to track the location of CME-driven shocks and the 3-D topology of open field lines along which flow particles produced by solar flares. Each of the four instrument packages produce a small real-time stream of selected data for purposes of predicting space weather events at Earth. NOAA forecasters at the Space Environment Center and others will use these data in their space weather forecasting and their resultant products will be widely used throughout the world. In addition to the four instrument teams, there is substantial participation by modeling and theory oriented teams. All STEREO data are freely available through individual Web sites at the four Principal Investigator institutions as well as at the STEREO Science Center located at NASA Goddard Space Flight Center.

A catalog of white light coronal mass ejections observed by the SOHO spacecraft

Abstract: [1] The Solar and Heliospheric Observatory (SOHO) mission's white light coronagraphs have observed nearly 7000 coronal mass ejections (CMEs) between 1996 and 2002. We have documented the measured properties of all these CMEs in an online catalog. We describe this catalog and present a summary of the statistical properties of the CMEs. The primary measurements made on each CME are the apparent central position angle, the angular width in the sky plane, and the height (heliocentric distance) as a function of time. The height-time measurements are then fitted to first- and second-order polynomials to derive the average apparent speed and acceleration of the CMEs. The statistical properties of CMEs are (1) the average width of normal CMEs (20° 900 km s−1) show deceleration. Solar cycle variation and statistical properties of CMEs are revealed with greater clarity in this study as compared with previous studies. Implications of our findings for CME models are discussed.

The Solar Probe Plus Mission: Humanity’s First Visit to Our Star

Abstract: Solar Probe Plus (SPP) will be the first spacecraft to fly into the low solar corona. SPP’s main science goal is to determine the structure and dynamics of the Sun’s coronal magnetic field, understand how the solar corona and wind are heated and accelerated, and determine what processes accelerate energetic particles. Understanding these fundamental phenomena has been a top-priority science goal for over five decades, dating back to the 1958 Simpson Committee Report. The scale and concept of such a mission has been revised at intervals since that time, yet the core has always been a close encounter with the Sun. The mission design and the technology and engineering developments enable SPP to meet its science objectives to: (1) Trace the flow of energy that heats and accelerates the solar corona and solar wind; (2) Determine the structure and dynamics of the plasma and magnetic fields at the sources of the solar wind; and (3) Explore mechanisms that accelerate and transport energetic particles. The SPP mission was confirmed in March 2014 and is under development as a part of NASA’s Living with a Star (LWS) Program. SPP is scheduled for launch in mid-2018, and will perform 24 orbits over a 7-year nominal mission duration. Seven Venus gravity assists gradually reduce SPP’s perihelion from 35 solar radii ( $R_{S}$ ) for the first orbit to ${<}10~R_{S}$ for the final three orbits. In this paper we present the science, mission concept and the baseline vehicle for SPP, and examine how the mission will address the key science questions

The solar flare myth

Abstract: Many years of research have demonstrated that large, nonrecurrent geomagnetic storms, shock wave disturbances in the solar wind, and energetic particle events in interplanetary space often occur in close association with large solar flares. This result has led to a pradigm of cause and effect - that large solar flares are the fundamental cause of these events in the near-Earth space environmemt. This paradigm, which I call 'the solar flare myth,' dominates the popular perception of the relationship between solar activity and interplanetary and geomagnetic events and has provided much of the pragmatic rationale for the study of the solar flare phenomenon. Yet there is good evidence that this paradigm is wrong and that flares do not generally play a central role in producing major transient disturbances in the near-Earth space environment. In this paper I outline a different paradigm of cause and effect that removes solar flares from their central position in the chain of events leading from the Sun to near-Earth space. Instead, this central role is given to events known as coronal mass ejections.