Level Upper Arc

The Level Upper Arc (編) or the Level Upper Incidents is a story arc that occurs within the Toaru Kagaku no Railgun manga and anime. It is the first story arc that occurs within the Toaru Majutsu no Index universe. It focuses on the Level Upper, a device that is sought out after by many people in Academy City as a means to easily gain powers. It is also a city-wide event that affected many espers and occurs during the middle of summer season, and canonically coincides with the beginning of the Index Arc.

The Level Upper
The Level Upper is a device or more specifically a sound that allows the users to gain abilities or make their current abilities better. Using, it stimulates the brain to modify it's brainwaves close to that of Kiyama Harumi, the developer of the Level Upper. Because espers unconsciously produce AIM, it is used as a common protocol among the espers with the same brain pattern making a connection which is called the Level Upper Network which is controlled by Harumi. With many people with the same brain patterns, the network has tremendous computational ability, users of the Level Upper can then compensate and gain or enhance abilities that otherwise gained through normal Academy City curriculum by using the computational abilities of the network. However, using the Level Upper causes the users to fall into a comatose state, as the result of having their brainwaves modified forcibly.

The Level Upper as an urban legend
The distribution of Level Upper has become widespread enough for people to have taken notice and use it, however it was not enough for people to take it seriously. The existence of Level Upper was simply an urban legend propagated by people and websites on the Internet, though this may have caused interest in people and may have made them seek and confirm the existence of the Level Upper themselves.

Criminal activity increases in Academy City
Before it's existence being confirmed by the authorities, many espers of Academy City have already used it and have now gained ability. Criminal activity has increased as a consequence of having, often low level espers, especially delinquents, gain abilities in a short period of time.

The Graviton Bombings
Kaitabi Hatsuya, a level 2 esper uses the Level Upper to strengthen his "Graviton" ability. Using aluminum materials stuffed inside mundane objects such as dolls, he creates miniature bombs that causes massive amounts of damage. Before his capture, Hatsuya injured nine members of Judgment and has caused massive amounts of damage in establishments such as Green Mart. His last bombing occurred in the Seventh Mist mall, using the highest output of his powers. The bombing made significant damage in a clothes boutique of the mall, but made no casualties due to the effort of Kamijou Touma. Hatsuya was later taken into custody by Shirai Kuroko with the aid of Misaka Mikoto.

Confirming the existence of the Level Upper
The power produced by Hatsuya's bombs created discrepancies in the databanks, as his level was insufficient to produced that much power. Judgment, following the rumors of a device that increases the level of an esper in a short period, they confirm its existence but are unable to obtain the Level Upper.. It is later discovered that many espers such as Kaitabi Hatsuya has collapsed and has become unconscious, they are later confirmed to have been users of the Level Upper.

Crackdown on the Level Upper
The prevention of the spreading of the Level Upper, the recovery of the comatose victims, and the arrest of the developer of the Level Upper are the top three priorities of Judgment as highlighted by Kuroko. During this point in time the use of the Level Upper has become widespread, and a considerable amount of criminals using their newly gained powers have also increased which strained the authorities considerably. Judgment has also taken protective custody of its user due to it's side-effects.

End of the Level Upper Incidents
Saten Ruiko, a close friend of Judgment member Uiharu Kazari collapses after using the Level Upper. In the anime her collapse forces Misaka Mikoto, the third level 5 esper to aid Judgment, as she blames herself for being insensitive to the feelings of its users, especially Ruiko who is a level 0, that was told by Mikoto that levels and abilities are nothing. With the help of the Heaven Canceller, Judgment figured out that the brainwaves of the Level Upper users all have the same pattern and are able to pinpoint Kiyama Harumi as the culprit behind the Level Upper. Mikoto goes after Harumi, Kuroko stays behind as she is still recovering after her encounter with Trick and other Level Upper users.

At the same time as this, Uiharu Kazari is kidnapped by Harumi after discovering her plans, she gives her the treatment program for the Level Upper and tells her the reason why she developed the Level Upper. Harumi is later forced to stop the car because of ann Anti-Skill barricade. The Anti-Skills demand Harumi to surrender but is surprised by Harumi having abilities, they are easily overwhelmed by her powers and are defeated.

Misaka Mikoto vs. Kiyama Harumi
Misaka Mikoto reaches Harumi and forces to her into a battle, initially Harumi had the upperhand as she controlled many abilities at the same time. However, she is tricked by Misaka Mikoto and is electrocuted by her, defeating her. Mikoto electrocuting Harumi allowed her to access her memories and the true reason why she developed the Level Upper.

Misaka Mikoto vs. The AIM Burst
Harumi regains consciousness and suddenly loses control of the Level Upper Network, this causes the birth of an accumulation of the Level Upper user's AIM diffusion, an AIM Burst. Mikoto attacked the being but it regenerates and becomes larger. All the while, the comatose victims of the Level Upper experience sudden convulsions after the AIM Burst manifested itself. After saving Tessou Tsuzuri from it, Mikoto is then told that the AIM Burst is heading for a nuclear testing facility which makes the situation even more dire. Harumi aids Mikoto by telling her that the Level Upper treatment program can help stop the AIM Burst. Uiharu Kazari with the help of Tsuzuri and Yomikawa Aiho plays the program(as it is a sound file as well) all over the city and calms the comatose victims down which stops the AIM Burst from regenerating. Misaka Mikoto is then able to use her railgun to destroy it's core, destroying the AIM Burst in the process.

Aftermath
Kiyama Harumi is apprehended by Anti-Skill shortly after the battle, she is however still determined to cure her students and gladly challenges Mikoto that she is welcome to try and stop her again. All comatose users of the Level Upper have began regaining consciousness as well. Many users of the Level Upper are given special lessons, here they learn the basics of ability development and are trained to surpass their limits.

List of Notable Level Upper users
Kiyama Harumi estimated that Level Upper users number around 10,000, which to her was a sufficient enough replacement for the Tree Diagram.

The list of notable users below are listed in alphabetical order.

Trivia

 * In the Toaru Kagaku no Railgun manga, the blackout caused by Mikoto due to fighting Touma on a bridge after failing to get information on the Level Upper is the same event that appears in the beginning of the Index Arc in Toaru Majutsu no Index. This event was completely changed in the anime adaptation as Mikoto caused the blackout while fighting Elder Sister, the anime still indicates that Mikoto fought Touma on a bridge which implies that in the anime's timeline, there were two blackouts that were caused by Mikoto.
 * In the Toaru Kagaku no Raikgun manga, it showed the reason why other students used the level upper, but in the anime, it only showed Saten's reason for using the level upper.