Leon Golub
Harvard University
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Featured researches published by Leon Golub.
Science | 2007
Jonathan Cirtain; Leon Golub; Loraine Louise Lundquist; A. A. van Ballegooijen; Antonia Savcheva; Masumi Shimojo; E. E. DeLuca; Saku Tsuneta; Taro Sakao; Kathy K. Reeves; Mark Alan Weber; R. Kano; Noriyuki Narukage; Kiyoto Shibasaki
Coronal magnetic fields are dynamic, and field lines may misalign, reassemble, and release energy by means of magnetic reconnection. Giant releases may generate solar flares and coronal mass ejections and, on a smaller scale, produce x-ray jets. Hinode observations of polar coronal holes reveal that x-ray jets have two distinct velocities: one near the Alfvén speed (∼800 kilometers per second) and another near the sound speed (200 kilometers per second). Many more jets were seen than have been reported previously; we detected an average of 10 events per hour up to these speeds, whereas previous observations documented only a handful per day with lower average speeds of 200 kilometers per second. The x-ray jets are about 2 × 103 to 2 × 104 kilometers wide and 1 × 105 kilometers long and last from 100 to 2500 seconds. The large number of events, coupled with the high velocities of the apparent outflows, indicates that the jets may contribute to the high-speed solar wind.
Nature | 2013
Jonathan Cirtain; Leon Golub; Amy R. Winebarger; B. De Pontieu; Ken Kobayashi; Ronald L. Moore; Robert William Walsh; Kelly Elizabeth Korreck; Mark Alan Weber; Patrick I. McCauley; A. M. Title; Sergei Kuzin; C. E. DeForest
It is now apparent that there are at least two heating mechanisms in the Sun’s outer atmosphere, or corona. Wave heating may be the prevalent mechanism in quiet solar periods and may contribute to heating the corona to 1,500,000 K (refs 1, 2, 3). The active corona needs additional heating to reach 2,000,000–4,000,000 K; this heat has been theoretically proposed to come from the reconnection and unravelling of magnetic ‘braids’. Evidence favouring that process has been inferred, but has not been generally accepted because observations are sparse and, in general, the braided magnetic strands that are thought to have an angular width of about 0.2 arc seconds have not been resolved. Fine-scale braiding has been seen in the chromosphere but not, until now, in the corona. Here we report observations, at a resolution of 0.2 arc seconds, of magnetic braids in a coronal active region that are reconnecting, relaxing and dissipating sufficient energy to heat the structures to about 4,000,000 K. Although our 5-minute observations cannot unambiguously identify the field reconnection and subsequent relaxation as the dominant heating mechanism throughout active regions, the energy available from the observed field relaxation in our example is ample for the observed heating.
Science | 2007
Taro Sakao; Ryouhei Kano; Noriyuki Narukage; Jun'ichi Kotoku; Takamasa Bando; Edward E. DeLuca; Loraine Louise Lundquist; Saku Tsuneta; Louise K. Harra; Yukio Katsukawa; Masahito Kubo; Hirohisa Hara; Keiichi Matsuzaki; Masumi Shimojo; Jay A. Bookbinder; Leon Golub; Kelly Elizabeth Korreck; Yingna Su; Kiyoto Shibasaki; Toshifumi Shimizu; Ichiro Nakatani
The Sun continuously expels a huge amount of ionized material into interplanetary space as the solar wind. Despite its influence on the heliospheric environment, the origin of the solar wind has yet to be well identified. In this paper, we report Hinode X-ray Telescope observations of a solar active region. At the edge of the active region, located adjacent to a coronal hole, a pattern of continuous outflow of soft-x-ray–emitting plasmas was identified emanating along apparently open magnetic field lines and into the upper corona. Estimates of temperature and density for the outflowing plasmas suggest a mass loss rate that amounts to ∼1/4 of the total mass loss rate of the solar wind. These outflows may be indicative of one of the solar wind sources at the Sun.
Solar Physics | 1977
Leon Golub; A. S. Krieger; J. W. Harvey; G. S. Vaiana
Using high resolution KPNO magnetograms and sequences of simultaneous S-054 soft X-ray solar images we have compared the properties of X-ray bright points (XBP) and ephemeral active regions (ER). All XBP appear on the magnetograms as bipolar features, except for very newly emerged or old and decayed XBP. We find that the separation of the magnetic bipoles increases with the age of the XBP, with an average emergence growth rate of 2.2 ± 0.4 km s−1. The total magnetic flux in a typical XBP living about 8 hr is found to be ≈ 2 x 1019 Mx. A proportionality is found between XBP lifetime and total magnetic flux, equivalent to ≈ 1020 Mx per day of lifetime.
The Astrophysical Journal | 1999
Dawn D. Lenz; Edward E. DeLuca; Leon Golub; R. Rosner; Jay A. Bookbinder
We report an initial study of temperature and emission-measure distributions along four steady loops observed with the Transition Region and Coronal Explorer at the limb of the Sun. The temperature diagnostic is the filter ratio of the extreme-ultraviolet 171 and 195 A passbands. The emission-measure diagnostic is the count rate in the 171 A passband. We find essentially no temperature variation along the loops. We compare the observed loop structure with theoretical isothermal and nonisothermal static loop structure.
Physics of Plasmas | 1999
Leon Golub; Jay A. Bookbinder; E. E. DeLuca; Margarita Karovska; H.P. Warren; Carolus J. Schrijver; R. A. Shine; Theodore D. Tarbell; Alan M. Title; J. Wolfson; Brian Neal Handy; Charles C. Kankelborg
The TRACE Observatory is the first solar-observing satellite in the National Aeronautics and Space Administration’s (NASA) Small Explorer series. Launched April 2, 1998, it is providing views of the solar transition region and low corona with unprecedented spatial and temporal resolution. The corona is now seen to be highly filamented, and filled with flows and other dynamic processes. Structure is seen down to the resolution limit of the instrument, while variability and motions are observed at all spatial locations in the solar atmosphere, and on very short time scales. Flares and shock waves are observed, and the formation of long-lived coronal structures, with consequent implications for coronal heating models, has been seen. This overview describes the instrument and presents some preliminary results from the first six months of operation.
The Astrophysical Journal | 1985
J. H. M. M. Schmitt; Leon Golub; F. R. Harnden; C. W. Maxson; R. Rosner; G. S. Vaiana
The results of an X-ray survey of bright late A and early F stars on the main B-V sequence between 0.1 and 0.5 are presented. All the stars were observed with the Einstein Observatory for a period of at least 500 seconds. The survey results show significantly larger X-ray luminosities for the sample binaries than for the single stars. It is suggested that the difference is due to the presence of multiple X-ray sources in binaries. It is shown that the X-ray luminosities for single stars increase rapidly with increasing color, and that the relation Lx/Lbol is equal to about 10 to the -7th does not hold for A stars. No correlation was found between X-ray luminosity and projected equatorial rotation velocity. It is argued on the basis of the observations that X-ray emission in the sample stars originated from coronae. The available observational evidence supporting this view is discussed.
The Astrophysical Journal | 2011
Suli Ma; John C. Raymond; Leon Golub; Jun Lin; Huadong Chen; Paolo C. Grigis; Paola Testa; David M. Long
Taking advantage of both the high temporal and spatial resolutions of the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, we studied a limb coronal shock wave and its associated extreme ultraviolet (EUV) wave that occurred on 2010 June 13. Our main findings are: (1) the shock wave appeared clearly only in the channels centered at 193 angstrom and 211 angstrom as a dome-like enhancement propagating ahead of its associated semi-spherical coronal mass ejection (CME) bubble; (2) the density compression of the shock is 1.56 according to radio data and the temperature of the shock is around 2.8 MK; (3) the shock wave first appeared at 05: 38 UT, 2 minutes after the associated flare has started and 1 minute after its associated CME bubble appeared; (4) the top of the dome-like shock wave set out from about 1.23 R-circle dot and the thickness of the shocked layer is similar to 2 x 10(4) km; (5) the speed of the shock wave is consistent with a slight decrease from about 600 km s(-1) to 550 km s(-1); and (6) the lateral expansion of the shock wave suggests a constant speed around 400 km s(-1), which varies at different heights and directions. Our findings support the view that the coronal shock wave is driven by the CME bubble, and the on-limb EUV wave is consistent with a fast wave or at least includes the fast wave component.
Science | 2014
Hui Tian; E. E. DeLuca; Steven R. Cranmer; B. De Pontieu; Hardi Peter; Juan Martinez-Sykora; Leon Golub; S. McKillop; K. K. Reeves; Mari Paz Miralles; Patrick I. McCauley; S. Saar; Paola Testa; Mark Alan Weber; Nicholas A. Murphy; James R. Lemen; A. M. Title; P. F. X. Boerner; N. Hurlburt; Theodore D. Tarbell; J.-P. Wuelser; Lucia Kleint; Charles C. Kankelborg; S. Jaeggli; Mats Carlsson; Viggo H. Hansteen; Scott W. McIntosh
As the interface between the Sun’s photosphere and corona, the chromosphere and transition region play a key role in the formation and acceleration of the solar wind. Observations from the Interface Region Imaging Spectrograph reveal the prevalence of intermittent small-scale jets with speeds of 80 to 250 kilometers per second from the narrow bright network lanes of this interface region. These jets have lifetimes of 20 to 80 seconds and widths of ≤300 kilometers. They originate from small-scale bright regions, often preceded by footpoint brightenings and accompanied by transverse waves with amplitudes of ~20 kilometers per second. Many jets reach temperatures of at least ~105 kelvin and constitute an important element of the transition region structures. They are likely an intermittent but persistent source of mass and energy for the solar wind.
Science | 2014
Hardi Peter; Hui Tian; W. Curdt; Donald Schmit; D. E. Innes; B. De Pontieu; James R. Lemen; A. M. Title; P. F. X. Boerner; N. Hurlburt; Theodore D. Tarbell; J.-P. Wuelser; Juan Martinez-Sykora; Lucia Kleint; Leon Golub; S. McKillop; K. K. Reeves; S. Saar; Paola Testa; Charles C. Kankelborg; S. Jaeggli; Mats Carlsson; Viggo H. Hansteen
The solar atmosphere was traditionally represented with a simple one-dimensional model. Over the past few decades, this paradigm shifted for the chromosphere and corona that constitute the outer atmosphere, which is now considered a dynamic structured envelope. Recent observations by the Interface Region Imaging Spectrograph (IRIS) reveal that it is difficult to determine what is up and down, even in the cool 6000-kelvin photosphere just above the solar surface: This region hosts pockets of hot plasma transiently heated to almost 100,000 kelvin. The energy to heat and accelerate the plasma requires a considerable fraction of the energy from flares, the largest solar disruptions. These IRIS observations not only confirm that the photosphere is more complex than conventionally thought, but also provide insight into the energy conversion in the process of magnetic reconnection.