حزام إشعاع ڤان ألن

(تم التحويل من Van Allen radiation belt)
هذا الفيديو يمثل التغيرات في شكل وشدة قطاع عرضي من أحزمة ڤان ألن.
Van Allen radiation belts
جزء من سلسلة مقالات عن
الفيزياء الشمسية
Heliospheric current sheet
Heliospheric current sheet
الأساسيات
Interplanetary medium
Magnetospherics
الظواهر

أحزمة إشعاع ڤان ألن، نطاقان من الجسيمات المشحونة كهربائيًا تحيطان بالسطح الأعلى للكرة الأرضية. وتُسمى أيضاً الأحزمة الإشعاعية. وقد سميت الأحزمة باسم مكتشفها جيمس ألفرد ڤان ألن، عالم الفيزياء الأمريكي الذي اكتشفها في عام 1958.

يتكوّن الإشعاع الموجود بهذه الأحزمة من تركيز عال من الجسيمات المشحونة بالكهرباء مثل البروتونات والإلكترونات. ويجذب المجال المغنطيسي الأرضي هذه الجسيمات ويوجهها نحو الأقطاب المغنطيسية. وتدور الجسيمات المنجذبة دوراناً لولبياً على خطوط وهمية في المجال المغنطيسي. وتنحرف خطوط المجال الوهمية هذه من القطب المغنطيسي الشمالي إلى القطب المغنطيسي الجنوبي. وعندما تقترب الجسيمات من أحد القطبين تعترضها خطوط المجال الوهمية وتعكسها نحو القطب الآخر. وهذه الظاهرة تجعل الجسيمات داخل الأحزمة تتأرجح بين الأقطاب.

الاكتشاف

اكتشف الفيزيائي الأمريكي جيمس ڤان ألن عام 1958 خلال عمله في رصد معلومات عن الإشعاعات الكونية بوساطة عداد گايگر-مولر G-M المحمول على القمرين الصناعيين إكسبلورر وبايونير. وتحيط الأحزمة بالكرة الأرضية إحاطة السوار بالمعصم، حيث يرتفع الحزام الداخلي بين حوالي 1,000 و5,000 كم فوق سطح الأرض، وقدّر ارتفاع الكثافة الإشعاعية العظمى عند 6000كم، الذي يقارب نصف قطر الأرض. تلا ذلك إرسال سواتل أخرى فاكتشف حزام إشعاعات آخر سمي الحزام الإشعاعي الخارجي. كما اكتشفت مؤخراً أحزمة أخرى تتشكل وتختفي. يمتد الحزام الأول ما بين 1.1 و3.3 مرة نصف قطر الأرض ويحتوي على بروتونات و أيونات أكسجين ونوى هليوم أو جسيمات ألفا، بينما يمتد الخارجي ما بين 3 و9 مرات نصف قطر الأرض ويشغل معظمه إلكترونات تصل ذروة كثافتها عند ارتفاع يساوي 4 مرات نصف قطر الأرض. ويتصف الحزام الأول باستقرار عالٍ بمرور الزمن بالمقارنة مع الاستقرار الضعيف للحزام الخارجي الذي يتغير تغيراً كبيراً وخاصة عند ذروة نشاط البقع الشمسية. فقد وجد أ ن مصدر معظم الأيونات عالية الطاقة هو الشمس إضافة إلى نواتج تفاعلات الأشعة الكونية مع الغلاف الجوي الأرضي.

Kristian Birkeland, Carl Størmer, Nicholas Christofilos, and Enrico Medi had investigated the possibility of trapped charged particles in 1895, forming a theoretical basis for the formation of radiation belts.[1] The second Soviet satellite Sputnik 2 which had detectors designed by Sergei Vernov,[2] followed by the US satellites Explorer 1 and Explorer 3,[3] confirmed the existence of the belt in early 1958, later named after James Van Allen from the University of Iowa.[4] The trapped radiation was first mapped by Explorer 4, Pioneer 3, and Luna 1.

The term Van Allen belts refers specifically to the radiation belts surrounding Earth; however, similar radiation belts have been discovered around other planets. The Sun does not support long-term radiation belts, as it lacks a stable, global dipole field. The Earth's atmosphere limits the belts' particles to regions above 200–1,000 km,[5] (124–620 miles) while the belts do not extend past 8 Earth radii RE.[5] The belts are confined to a volume which extends about 65°[5] on either side of the celestial equator.

أبحاث

Jupiter's variable radiation belts

The NASA Van Allen Probes mission aims at understanding (to the point of predictability) how populations of relativistic electrons and ions in space form or change in response to changes in solar activity and the solar wind. NASA Institute for Advanced Concepts–funded studies have proposed magnetic scoops to collect antimatter that naturally occurs in the Van Allen belts of Earth, although only about 10 micrograms of antiprotons are estimated to exist in the entire belt.[6]

The Van Allen Probes mission successfully launched on August 30, 2012. The primary mission was scheduled to last two years with expendables expected to last four. The probes were deactivated in 2019 after running out of fuel and are expected to deorbit during the 2030s.[7] NASA's Goddard Space Flight Center manages the Living With a Star program—of which the Van Allen Probes were a project, along with Solar Dynamics Observatory (SDO). The Applied Physics Laboratory was responsible for the implementation and instrument management for the Van Allen Probes.[8]

Radiation belts exist around other planets and moons in the Solar System that have magnetic fields powerful and stable enough to sustain them. Radiation belts have been detected at Jupiter, Saturn, Uranus and Neptune through in-situ observations, such as by the Galileo and Juno spacecraft at Jupiter, Cassini–Huygens at Saturn, and fly-bys from the Voyager program and Pioneer program. Observations of radio emissions from highly energetic particles that are trapped in a planets magnetic field have also been used to remotely detect radiation belts, including at Jupiter [9] and at the ultracool dwarf LSR J1835+3259.[10] It is possible that Mercury (planet) may be able to trap charged particles in its magnetic field,[11] although its highly dynamic magnetosphere (which varies on the order of minutes [12]) may not be able to sustain stable radiation belts. Venus and Mars do not have radiation belts, as their magnetospheric configurations do not trap energetic charged particles in orbit around the planet.

Geomagnetic storms can cause electron density to increase or decrease relatively quickly (i.e., approximately one day or less). Longer-timescale processes determine the overall configuration of the belts. After electron injection increases electron density, electron density is often observed to decay exponentially. Those decay time constants are called "lifetimes." Measurements from the Van Allen Probe B's Magnetic Electron Ion Spectrometer (MagEIS) show long electron lifetimes (i.e., longer than 100 days) in the inner belt; short electron lifetimes of around one or two days are observed in the "slot" between the belts; and energy-dependent electron lifetimes of roughly five to 20 days are found in the outer belt.[13]

الحزام الداخلي

يجذب الحزام الداخلي الجسيمات المنطلقة من الغلاف الجوي بوساطة أشعة كونية وهي جسيمات ذات طاقة عالية في الفضاء الخارجي.

Cutaway drawing of two radiation belts around Earth: the inner belt (red) dominated by protons and the outer one (blue) by electrons. Image Credit: NASA

The inner Van Allen Belt extends typically from an altitude of 0.2 to 2 Earth radii (L values of 1.2 to 3) or 1،000 km (620 mi) to 12،000 km (7،500 mi) above the Earth.[14][15] In certain cases, when solar activity is stronger or in geographical areas such as the South Atlantic Anomaly, the inner boundary may decline to roughly 200 km[16] above the Earth's surface. The inner belt contains high concentrations of electrons in the range of hundreds of keV and energetic protons with energies exceeding 100 MeV—trapped by the relatively strong magnetic fields in the region (as compared to the outer belt).[17]

It is thought that proton energies exceeding 50 MeV in the lower belts at lower altitudes are the result of the beta decay of neutrons created by cosmic ray collisions with nuclei of the upper atmosphere. The source of lower energy protons is believed to be proton diffusion, due to changes in the magnetic field during geomagnetic storms.[18]

Due to the slight offset of the belts from Earth's geometric center, the inner Van Allen belt makes its closest approach to the surface at the South Atlantic Anomaly.[19] [20]

In March 2014, a pattern resembling "zebra stripes" was observed in the radiation belts by the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) onboard Van Allen Probes. The initial theory proposed in 2014 was that—due to the tilt in Earth's magnetic field axis—the planet's rotation generated an oscillating, weak electric field that permeates through the entire inner radiation belt.[21] A 2016 study instead concluded that the zebra stripes were an imprint of ionospheric winds on radiation belts.[22]

الحزام الخارجي

Laboratory simulation of the Van Allen belts' influence on the Solar Wind; these auroral-like Birkeland currents were created by the scientist Kristian Birkeland in his terrella, a magnetized anode globe in an evacuated chamber.

يجذب الحزام الخارجي الجسيمات من الرياح الشمسية، وهي جسيمات مشحونة بالكهرباء تتدفق باستمرار من الشمس، ومن اللهب الشمسي وهي انفجارات فجائية تحدث على سطح الشمس. ويمزق هذا النشاط الشمسي الكبير الأحزمة ويؤدي إلى عواصف مغنطيسية. وتتداخل اضطرابات الأحزمة أيضاً مع استقبال الراديو، وتسبب موجات في خطوط القدرة الكهربائية بالإضافة إلى تكوين الأورورات. انظر: أورورا.

وتحاط الكواكب الأخرى مثل المشتري وزُحل وأورانوس (سابع الكواكب السيارة) ونبتون أيضًا بمجالات مغنطيسية مثل كوكب الأرض. وقد أثبتت الرحلات الفضائية في العقدين الثامن والتاسع من القرن العشرين الميلادي، أن لهذه الكواكب أحزمة إشعاعية أيضاً.

The outer belt consists mainly of high-energy (0.1–10 MeV) electrons trapped by the Earth's magnetosphere. It is more variable than the inner belt, as it is more easily influenced by solar activity. It is almost toroidal in shape, beginning at an altitude of 3 Earth radii and extending to 10 Earth radii (RE)—13،000 إلى 60،000 كيلومتر (8،100 إلى 37،300 mi) above the Earth's surface.[بحاجة لمصدر] Its greatest intensity is usually around 4 to 5 RE. The outer electron radiation belt is mostly produced by inward radial diffusion[23][24] and local acceleration[25] due to transfer of energy from whistler-mode plasma waves to radiation belt electrons. Radiation belt electrons are also constantly removed by collisions with Earth's atmosphere,[25] losses to the magnetopause, and their outward radial diffusion. The gyroradii of energetic protons would be large enough to bring them into contact with the Earth's atmosphere. Within this belt, the electrons have a high flux and at the outer edge (close to the magnetopause), where geomagnetic field lines open into the geomagnetic "tail", the flux of energetic electrons can drop to the low interplanetary levels within about 100 km (62 mi)—a decrease by a factor of 1,000.

In 2014, it was discovered that the inner edge of the outer belt is characterized by a very sharp transition, below which highly relativistic electrons (> 5MeV) cannot penetrate.[26] The reason for this shield-like behavior is not well understood.

The trapped particle population of the outer belt is varied, containing electrons and various ions. Most of the ions are in the form of energetic protons, but a certain percentage are alpha particles and O+ oxygen ions—similar to those in the ionosphere but much more energetic. This mixture of ions suggests that ring current particles probably originate from more than one source.

The outer belt is larger than the inner belt, and its particle population fluctuates widely. Energetic (radiation) particle fluxes can increase and decrease dramatically in response to geomagnetic storms, which are themselves triggered by magnetic field and plasma disturbances produced by the Sun. The increases are due to storm-related injections and acceleration of particles from the tail of the magnetosphere. Another cause of variability of the outer belt particle populations is the wave-particle interactions with various plasma waves in a broad range of frequencies.[27]

On February 28, 2013, a third radiation belt—consisting of high-energy ultrarelativistic charged particles—was reported to be discovered. In a news conference by NASA's Van Allen Probe team, it was stated that this third belt is a product of coronal mass ejection from the Sun. It has been represented as a separate creation which splits the Outer Belt, like a knife, on its outer side, and exists separately as a storage container of particles for a month's time, before merging once again with the Outer Belt.[28]

The unusual stability of this third, transient belt has been explained as due to a 'trapping' by the Earth's magnetic field of ultrarelativistic particles as they are lost from the second, traditional outer belt. While the outer zone, which forms and disappears over a day, is highly variable due to interactions with the atmosphere, the ultrarelativistic particles of the third belt are thought not to scatter into the atmosphere, as they are too energetic to interact with atmospheric waves at low latitudes.[29] This absence of scattering and the trapping allows them to persist for a long time, finally only being destroyed by an unusual event, such as the shock wave from the Sun.

قيم الفيض

In the belts, at a given point, the flux of particles of a given energy decreases sharply with energy.

At the magnetic equator, electrons of energies exceeding 5000 keV (resp. 5 MeV) have omnidirectional fluxes ranging from 1.2×106 (resp. 3.7×104) up to 9.4×109 (resp. 2×107) particles per square centimeter per second.

The proton belts contain protons with kinetic energies ranging from about 100 keV, which can penetrate 0.6 μm of lead, to over 400 MeV, which can penetrate 143 mm of lead.[30]

Most published flux values for the inner and outer belts may not show the maximum probable flux densities that are possible in the belts. There is a reason for this discrepancy: the flux density and the location of the peak flux is variable, depending primarily on solar activity, and the number of spacecraft with instruments observing the belt in real time has been limited. The Earth has not yet experienced a solar storm of Carrington event intensity while spacecraft with the proper instruments have been available to observe the event.

Radiation levels in the belts would be dangerous to humans if they were exposed for an extended period of time. The Apollo missions minimised hazards for astronauts by sending spacecraft at high speeds through the thinner areas of the upper belts, bypassing inner belts completely, except for the Apollo 14 mission where the spacecraft traveled through the heart of the trapped radiation belts.[19][31][32][33]

  • Flux values, normal solar conditions
  • AP8 MIN omnidirectional proton flux ≥ 100 keV

    AP8 MIN omnidirectional proton flux ≥ 100 keV

  • AP8 MIN omnidirectional proton flux ≥ 1 MeV

    AP8 MIN omnidirectional proton flux ≥ 1 MeV

  • AP8 MIN omnidirectional proton flux ≥ 400 MeV

    AP8 MIN omnidirectional proton flux ≥ 400 MeV

احتجاز المادة المضادة

In 2011, a study confirmed earlier speculation that the Van Allen belt could confine antiparticles. The Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) experiment detected levels of antiprotons orders of magnitude higher than are expected from normal particle decays while passing through the South Atlantic Anomaly. This suggests the Van Allen belts confine a significant flux of antiprotons produced by the interaction of the Earth's upper atmosphere with cosmic rays.[34] The energy of the antiprotons has been measured in the range from 60 to 750 MeV.

The very high energy released in antimatter annihilation has led to proposals to harness these antiprotons for spacecraft propulsion. The concept relies on the development of antimatter collectors and containers.[35]

تأثيره على السفر في الفضاء

مقارنة أحجام مدارات GPS و گلوناس و گاليليو و بـِيْ‌دو-2 و إيريديوم، بمدار كلٍ من محطة الفضاء الدولية وتلسكوب هبل الفضائي و المدار الثابت بالنسبة للأرض (ومدار الجبانة له)، مع أحزمة إشعاع ڤان ألن والأرض بنفس مقياس الرسم.[أ]
مدار القمر هو حوالي 9 أضعاف المدار الثابت بالنسبة للأرض.[ب] (في ملف SVG، حـُمْ فوق مدار أو فوق عنوانه لتبيانه؛ انقر لتحميل مقاله.)

Spacecraft travelling beyond low Earth orbit enter the zone of radiation of the Van Allen belts. Beyond the belts, they face additional hazards from cosmic rays and solar particle events. A region between the inner and outer Van Allen belts lies at 2 to 4 Earth radii and is sometimes referred to as the "safe zone".[36][37]

Solar cells, integrated circuits, and sensors can be damaged by radiation. Geomagnetic storms occasionally damage electronic components on spacecraft. Miniaturization and digitization of electronics and logic circuits have made satellites more vulnerable to radiation, as the total electric charge in these circuits is now small enough so as to be comparable with the charge of incoming ions. Electronics on satellites must be hardened against radiation to operate reliably. The Chandra Space Telescope, has its sensors turned off when passing through the Van Allen belts.[38] The INTEGRAL space telescope was placed in an orbit designed to avoid time with in the belts.[39]

The Apollo missions marked the first event where humans traveled through the Van Allen belts, which was one of several radiation hazards known by mission planners.[40] The astronauts had low exposure in the Van Allen belts due to the short period of time spent flying through them.[32][41]

مسبباته

Simulated Van Allen Belts generated by a plasma thruster in tank #5 Electric Propulsion Laboratory at the then-called Lewis Research Center, Cleveland, Ohio.

تعود أسباب تشكل الحزام إلى تفاعل بين الجسيمات المشحونة والحقل المغنطيسي فقد تحدث العلماء قبل عام 1958 عن إمكان أسر الأيونات والإلكترونات بوساطة حقول مغنطيسية، إذ يؤثر الحقل المغنطيسي الموضعي الذي يمثل بخطوط الحقل، في الجسيمات المشحونة السريعة فيجعلها ترسم مساراً لولبياً حول هذه الخطوط، كما يمكن أن ترتحل من خط لآخر عند تغير هذه الحقول. ويعتمد التأثير على شدة الحقل، أو على تراص خطوطه، إلى درجة يمكن عندها أن تجعل الجسيمات المشحونة تتباطأ أو تعود من حيث أتت. يحدث أمر كهذا عند القطبين المغنطيسيين للأرض اللذين يعملان عمل مرآتين مغنطيسيتين كالتي تستعمل في أبحاث تحريك السوائل المغنطيسي. وقد أمكن شرح أشكال الأحزمة انطلاقاً من أشكال خطوط الحقل المغنطيسي الأرضي و ما يطرأ عليها من تبدلات وخاصة عند هبوب الرياح الشمسية. يبدو في الصورة المرافقة حزامان إشعاعيان يحيطان بالكرة الأرضية.

الإزالة المقترحة

Draining the charged particles from the Van Allen belts would open up new orbits for satellites and make travel safer for astronauts.[42]

High Voltage Orbiting Long Tether, or HiVOLT, is a concept proposed by Russian physicist V. V. Danilov and further refined by Robert P. Hoyt and Robert L. Forward for draining and removing the radiation fields of the Van Allen radiation belts[43] that surround the Earth.[44]

Another proposal for draining the Van Allen belts involves beaming very-low-frequency (VLF) radio waves from the ground into the Van Allen belts.[45]

Draining radiation belts around other planets has also been proposed, for example, before exploring Europa, which orbits within Jupiter's radiation belt.[46]

As of 2024, it remains uncertain if there are any negative unintended consequences to removing these radiation belts.[42]

أضراره

يتعدى الاهتمام الحالي بالأحزمة الإشعاعية الفضول العلمي ليصل إلى ما يمكن أن تسببه من أضرار، شأنها شأن الإشعاعات الكونية الأخرى، للسواتل وما تحمله من دارات إلكترونية ولوحات شمسية فوتوفولطائية بل حتى لرواد الفضاء والسفن الفضائية الأخرى. كما تؤثر تغيراتها في الاتصالات وجودتها متمثلة في الضجيج و الأضرار التي تلحقها بالحواسيب التي تحملها السفن الفضائية وغيرها.[47]

انظر أيضاً

المصادر

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