Explanation

AURORA

            An aurora (plural: aurorae or auroras; from the Latin word aurora, "sunrise" or the Roman goddess of dawn) is a natural light display in the sky particularly in the high latitude (Arctic and Antarctic) regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere (thermosphere). The charged particles originate in the magnetosphere and solar wind and, on Earth, are directed by the Earth's magnetic field into the atmosphere. Most aurorae occur in a band known as the auroral zone,^[1][2] which is typically 3° to 6° in latitudinal extent and at all local times or longitudes. The auroral zone is typically 10° to 20° from the magnetic pole defined by the axis of the Earth's magnetic dipole. During a geomagnetic storm, the auroral zone expands to lower latitudes.

           Aurorae are classified as diffuse and discrete. The diffuse aurora is a featureless glow in the sky that may not be visible to the naked eye, even on a dark night. It defines the extent of the auroral zone. The discrete aurorae are sharply defined features within the diffuse aurora that vary in brightness from just barely visible to the naked eye, to bright enough to read a newspaper by at night. Discrete aurorae are usually seen in only the night sky, because they are not as bright as the sunlit sky. Aurorae occasionally occur poleward of the auroral zone as diffuse patches or arcs,^[3] which are generally subvisual.

           In northern latitudes, the effect is known as the aurora borealis (or the northern lights), named after the Roman goddess of dawn, Aurora, and the Greek name for the north wind, Boreas, by Pierre Gassendi in 1621.^[4] Auroras seen near the magnetic pole may be high overhead, but from farther away, they illuminate the northern horizon as a greenish glow or sometimes a faint red, as if the Sun were rising from an unusual direction. Discrete aurorae often display magnetic field lines or curtain-like structures, and can change within seconds or glow unchanging for hours, most often in fluorescent green. The aurora borealis most often occurs near the equinoxes. The northern lights have had a number of names throughout history. The Cree call this phenomenon the "Dance of the Spirits". In Medieval Europe, the auroras were commonly believed to be a sign from God.^[5]

              Its southern counterpart, the aurora australis (or the southern lights), has features that are almost identical to the aurora borealis and changes simultaneously with changes in the northern auroral zone.^[6] It is visible from high southern latitudes in Antarctica, South America, New Zealand, and Australia. Aurorae occur on other planets. Similar to the Earth's aurora, they are visible close to the planet's magnetic poles. Modern style guides recommend that the names of meteorological phenomena, such as aurora borealis, be uncapitalized.^[7]

             Typically the aurora appears either as a diffuse glow or as "curtains" that approximately extend in the east-west direction. At some times, they form "quiet arcs"; at others ("active aurora"), they evolve and change constantly. Each curtain consists of many parallel rays, each lined up with the local direction of the magnetic field lines, suggesting that auroras are shaped by Earth's magnetic field. Indeed, satellites show that electrons are guided by magnetic field lines, spiraling around them while moving toward Earth.

             The similarity to curtains is often enhanced by folds called "striations". When the field line guiding a bright auroral patch leads to a point directly above the observer, the aurora may appear as a "corona" of diverging rays, an effect of perspective.

Although it was first mentioned by Ancient Greek explorer/geographer Pytheas, Hiorter and Celsius first described in 1741 evidence for magnetic control, namely, large magnetic fluctuations occurred whenever the aurora was observed overhead. This indicates (it was later realized) that large electric currents were associated with the aurora, flowing in the region where auroral light originated. Kristian Birkeland (1908)^[13] deduced that the currents flowed in the east-west directions along the auroral arc, and such currents, flowing from the dayside toward (approximately) midnight were later named "auroral electrojets" (see also Birkeland currents).

              Still more evidence for a magnetic connection are the statistics of auroral observations. Elias Loomis (1860) and later in more detail Hermann Fritz (1881)^[14] and S. Tromholt (1882)^[15] established that the aurora appeared mainly in the "auroral zone", a ring-shaped region with a radius of approximately 2500 km around Earth's magnetic pole. It was hardly ever seen near the geographic pole, which is about 2000 km away from the magnetic pole. The instantaneous distribution of auroras ("auroral oval"^[1][2]) is slightly different, centered about 3–5 degrees nightward of the magnetic pole, so that auroral arcs reach furthest toward the equator about an hour before midnight. The aurora can be seen best at this time, called magnetic midnight, which occurs when an observer, the magnetic pole in question and the Sun are in alignment.

             In the 1970s, astrophysicist Joan Feynman deduced that auroras are a product of the interaction between the Earth's magnetosphere and the magnetic field of the solar wind.^[16] Her work resulted from data collected by the Explorer 33 spacecraft.^[17]

             On 26 February 2008, THEMIS probes were able to determine, for the first time, the triggering event for the onset of magnetospheric substorms.^[18] Two of the five probes, positioned approximately one third the distance to the moon, measured events suggesting a magnetic reconnection event 96 seconds prior to auroral intensification.^[19] Dr. Vassilis Angelopoulos of the University of California, Los Angeles, the principal investigator for the THEMIS mission, claimed, "Our data show clearly and for the first time that magnetic reconnection is the trigger."^[20]

^{source : file:///D:/Aurora%20%28astronomy%29%20-%20Wikipedia,%20the%20free%20encyclopedia.htm}

CERPEN

Study Tour

            Aku dan teman-temanku bersorak riang ketika kak Feri—guru bimbel bahasa inggrisku memberitahukan bahwa kami akan mengadakan study tour. Kami sudah mengira bahwa kami akan pergi mendaki gunung, ke pantai atau wisata alam lainnya. Tetapi kelas mendadak senyap ketika tahu tempat tujuan kami yaitu Museum.

            Kami saling menatap heran, bahkan Joseph—teman yang  duduk disampingku menatapku dengan mulutnya yang terbuka dan alisnya yang bertaut seakan mewakili pertanyaannya. Aku mrngangkat alis, bahu, tangan secara bersamaan untuk mengisyaratkan aku juga sama-sama bingung. Kak Feri  tertawa—seakan tebakan yang sudah ia kira atas nespon kami, tepat sasaran. Teman temanku semakin melongo,memperlihatkan wajah konyol mereka saat menjelaskan tujuan kami yaitu Museum Geologi.

            ‘’Disana kalian akan benar-benar  belajar.’’ Ucap kak Feri semakin memperburuk suasana hati kami.’’ Kalian akan bertemu dengan teman-teman dari Australia ,’’ lanjut  kak Feri yang jujur—sedikit membuatku penasaran. “Kalian diharuskan untuk berkenalan, mengobrol, menceritakan kota Bandung atau Indonesia seperti Tourguide dan tentu saja memakai Bahasa Inggris.” Akhirnya. Kalimat terakhir itu mengungkapkan kekhawatiran teman-temanku, termasuk aku. Bahasa Inggrisku payah sekali—mungkin ada di titik 5 dari skala 100.

            Kami mulai giat belajar dan berlatih. Aku sering belajar kelompok dengan teman-temanku. Kami menghapal kata-kata yang mungkin akan kami butuhkan. Kak Feri juga sering membantu kami. Akhirnya, hari minggu itu aku dan teman-teman sudah siap-siap di tempat bimbel menunggu mobil jemputan untuk pergi ke museum Geologi. Setelah sampai disana, kami mengikuti kak Feri dan kak Linna masuk ke dalam museum. Tak lama setelah mengurus tiket, kami pun masuk lebih ke dalam lagi sampai aku melihat kira-kira ada 20 lebih orang asing.

            “Yaampun, banyak sekali..” ucap Rei, salah satu temanku. Kami sama-sama terkejut. Aku kira mereka tidak akan lebih dari kami, tapi kenyataannya jumlah mereka dua kali lipat dari kami. Tiga dari mereka berumur lebig dari 20 tahun. Dua laki-laki dan satu perempuan yang kemudian mengobrol dengn guruku. Sedang sisanya yaitu anak laki-laki dan perempuan yang berumur sekitar 12-15 tahun, mayoritasnya perempuan.

            Kami semua diam sambil memperhatikan rombongan orang asing itu. Kak Linna menghampiri kami, “Jangan khawatir, semua akan baik-baik saja.” Ucapnya sambil tersenyum. Aku bingung harus melakukan apa. Keberanian yang kukumpulkan menguap bersama materi-materi hasil belajar. Tidak ada yang bergerak diantara kami. “Oh ya, kalian sekarang boleh berkeliling. Jangan lupa berkenalan dengan mereka oke? Good luck, guys!” tambahnya seakan menjawab pikiranku.

            Akhirnya kami berpencar, ada teman-temanku yang mulai memberanikan diri untuk berkenalan. Sedangkan aku hanya mengikuti langkah kakiku. Aku ikut rombongan paling banyak. Aku memperhatikan seorang anak laki-laki berambut cokelat yang sedang tertawa dengan Joseph. Joseph melihat kearahku, aku memasang wajah memelas—lalu dia tertawa. Aku ikut tertawa ketika seseorang menyapaku dari belakang.

            “Hai” sapanya. Aku menoleh—dan dengan gugup menjawab “Halo”. Seorang anak perempuan berambut hitam lurus, berkulit pucat dengan mata sipit. Ia lalu tersenyum sambil menjajari langkahku. Aku tak tahu harus bicara apa—lebih tepatnya aku tidak berani bicara. Jika aku salah bicara pasti konyol. ‘Nanti dia tidak mengerti lagi,’ pikirku sambil membayangkan jika itu terjadi. Memalukan.

            “Aku Akira. Hashizume Akira, kau?” pertanyaannya menghentikan pikiranku. Ia mengulurkan tangan. Dia berbicara bahasa Indonesia? Seakan baru sadar, aku cepat-cepat menjabat tangannya. “Kau berbicara.. Bahasa Indonesia?” Ia mengangguk sambil tersenyum tipis. Mulutku membentuk huruf o. Dia menatap wajahku seperti meminta jawaban. “Ahiya, aku Kianti.” Dia mengangguk. “Halo Kianti.” Aku tersenyum untuk menjawabnya.

            Akira sangat menyenangkan. Hari itu yang seharusnya aku yang banyak bicara, jadi terbalik. Akira menceritakan tentang keluarga, sekolah, dan lain-lain. Aku lebih sering menyimak dan sekali-kali memberi tanggapan. Aku bercerita hanya ketika Akira bertanya. Aku juga berkenalan dengan yang lainnya. Seperti Rene—anak perempuan yang sangat ceriwis, bicaranya cepat sekali sehingga aku hanya dapat menyimak beberapa kata. Ada juga Greg—yang pintar sekali, berwawasan luas dan tampan. Dan banyak lagi, aku lupa.

            Akira membantuku berkomunikasi dengan mereka, dia sangat fasih berbahasa Indonesia karena nenek dari ibunya adalah orang Indonesia. Mereka semua ramah dan menyenangkan. Kami bertukar cerita, bernyanyi dan berfoto. Hari itu adalah hari yang menyenangkan. Aku takkan melupakannya.